That is excellent! Getting the folder structure set up early is the best thing you can do for a project of this size.

For now, your tpm-style.css doesn't need to be massive. Because you are already using standard WordPress classes (like wp-list-table and button-primary), your theme is doing a lot of the heavy lifting.

However, we can use this stylesheet to set up a clean foundation, handle responsive table behaviors, and prepare to move away from inline styles (like style="color:red;") in the future.

Here is a great starter template for your tpm-style.css file. You can just copy and paste this in:

CSS
/* * TPM Portal Core Styles 
 * This file handles the frontend appearance of the Teacher Portal shortcode.
 */

/* --- Main Container --- */
#tpm-portal-container {
    max-width: 900px; /* Keeps the portal from stretching too wide on large screens */
    margin: 0 auto;
    padding: 15px 0;
}

/* --- Typography & Headers --- */
#tpm-portal-container h3, 
#tpm-portal-container h4 {
    color: #2271b1; /* WordPress admin blue */
}

/* --- Forms & Inputs --- */
#tpm-portal-container label {
    font-weight: bold;
    display: block;
    margin-bottom: 5px;
}

#tpm-portal-container input[type="email"],
#tpm-portal-container input[type="text"],
#tpm-portal-container input[type="date"],
#tpm-portal-container textarea {
    width: 100%;
    max-width: 400px; /* Keeps inputs readable */
    padding: 8px;
    margin-bottom: 15px;
    border: 1px solid #8c8f94;
    border-radius: 4px;
}

#tpm-portal-container textarea {
    max-width: 100%; /* Let textareas take full width */
}

/* --- Messages (Success / Error) --- */
.tpm-message {
    margin-top: 15px;
    font-weight: bold;
    padding: 10px;
    border-radius: 4px;
}

.tpm-error {
    color: #d63638; /* WP Admin Red */
    background: #fcf0f1;
    border-left: 4px solid #d63638;
}

.tpm-success {
    color: #00a32a; /* WP Admin Green */
    background: #f0f7f4;
    border-left: 4px solid #00a32a;
}

/* --- Step 3 Specifics --- */
.tpm-hours-grid {
    display: flex; 
    flex-direction: column; 
    gap: 8px; 
    background: #fff; 
    padding: 15px; 
    border: 1px solid #ddd; 
    border-radius: 4px;
    max-width: 400px;
}

/* =========================================
   DESKTOP TABLE STYLES
   ========================================= */
.tpm-classes-table {
    width: 100%;
    border-collapse: collapse;
    margin-top: 20px;
    background: #fff;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
}
.tpm-classes-table th, 
.tpm-classes-table td {
    padding: 12px 15px;
    text-align: left;
    border-bottom: 1px solid #e2e4e7;
}
.tpm-classes-table th {
    background-color: #f8f9fa;
    font-weight: 600;
    color: #3c434a;
}
.tpm-classes-table tbody tr:hover {
    background-color: #fcfcfc;
}

/* =========================================
   MOBILE CARD STYLES (Max width 768px)
   ========================================= */
@media screen and (max-width: 768px) {
    /* Hide the traditional table header */
    .tpm-classes-table thead {
        display: none; 
    }
    
    /* Turn rows into cards */
    .tpm-classes-table tbody tr {
        display: block;
        margin-bottom: 20px;
        border: 1px solid #c3c4c7;
        border-radius: 8px;
        padding: 15px;
        background: #fff;
        box-shadow: 0 2px 5px rgba(0,0,0,0.05);
    }
    
    /* Turn cells into flexbox rows inside the card */
    .tpm-classes-table tbody td {
        display: flex;
        justify-content: flex-start; /* Data justified left instead of spaced out */
        align-items: flex-start;     /* Top vertical alignment instead of center */
        padding: 10px 0;
        border-bottom: 1px solid #f0f0f1;
        text-align: left;
        gap: 10px; /* Adds a little breathing room between the label and the data */
    }
    
    /* Remove border from the last item (the button) */
	.tpm-classes-table tbody td:last-child {
		border-bottom: none;
		padding-bottom: 0;
		justify-content: center; /* center horizontally */
		margin-top: 15px;
	}
    
    /* Pull the header label from the data-label attribute! */
    .tpm-classes-table tbody td::before {
        content: attr(data-label) ":"; /* Added the colon character here */
        font-weight: 600;
        color: #50575e;
        text-align: left;
        flex-basis: 35%; /* Keeps all labels the same width so the data aligns in a neat column */
        flex-shrink: 0;  /* Prevents the label from squishing if the data text is long */
    }

    /* Hide the label for the button row */
    .tpm-classes-table tbody td:last-child::before {
        display: none; 
    }

    /* Button styling - no longer stretching */
    .tpm-classes-table tbody td button {
        width: auto; 
        padding: 8px 20px;
        font-size: 16px; 
    }
	
	.tpm-select-class {
		display: inline-flex;
		align-items: center;
		justify-content: center;
		gap: 8px; /* space between label and arrow */
	}

	.tpm-btn-label {
		font-weight: 500;
	}

	.tpm-btn-arrow {
		font-size: 16px;
	}	
}

@media screen and (min-width: 769px) {
    .tpm-btn-label {
        display: none;
    }
}


/* =========================================
   CASE SELECTOR (Step 3 Tiles)
   ========================================= */
.tpm-case-selector {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 15px;
}

@media screen and (max-width: 600px) {
    .tpm-case-selector {
        grid-template-columns: 1fr; /* 1 στήλη στα κινητά */
    }
}

/* =========================================
   CASE SELECTOR (Step 3 Tiles)
   ========================================= */
.tpm-case-selector {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 15px;
}

@media screen and (max-width: 600px) {
    .tpm-case-selector {
        grid-template-columns: 1fr;
    }
}

.tpm-tile-btn {
    display: block;
    width: 100%;
    padding: 20px 15px;
    background: #fff;
    border: 2px solid #ddd;
    border-radius: 8px;
    text-align: center;
    font-weight: 600;
    font-size: 16px;
    color: #50575e;
    cursor: pointer;
    transition: all 0.2s ease;
    box-sizing: border-box; /* Ensures padding doesn't break the grid */
}

/* The hover state now gets that bold blue "selected" look */
.tpm-tile-btn:hover {
    border-color: #2271b1;
    background: #f0f6fc;
    color: #2271b1;
    box-shadow: 0 0 0 1px #2271b1;
}

/* =========================================
   SUBSTITUTION HOURS (Step 4 B)
   ========================================= */
/* 1. Break the parent containers out of the theme's 2/3 restriction */
#tpm-modification-form, 
#tpm-makeup-fields, 
#tpm-form-fields-container {
    width: 100% !important;
    max-width: 100% !important; 
    box-sizing: border-box !important;
}

/* 2. The Grid Container - FORCED ROW */
.tpm-hours-grid {
    display: flex !important;
    flex-direction: row !important;  /* CRITICAL: Forces left-to-right flow, stopping the stack */
    flex-wrap: wrap !important;      
    justify-content: flex-start !important; 
    gap: 20px !important;
    width: 100% !important;
    max-width: 100% !important;      
    margin: 10px 0 20px 0 !important; 
    padding: 20px !important;          
    box-sizing: border-box !important;
}

/* 3. The 170px Tiles - HEIGHT CAPPED */
.tpm-hours-grid label {
    display: flex !important;          
    flex-direction: row !important;    /* Keeps the checkbox and text side-by-side */
    align-items: center !important;
    flex: 0 0 170px !important;        /* Width basis */
    width: 170px !important;           
    max-width: 170px !important;       
    height: auto !important;           /* CRITICAL: Kills the 170px vertical stretch */
    min-height: 36px !important;       /* Gives them a consistent, neat height */
    box-sizing: border-box !important; 
    background: #fff !important;
    padding: 8px 12px !important;      
    border: 1px solid #ccd0d4 !important;
    border-radius: 4px !important;
    cursor: pointer !important;
    font-size: 13px !important;     
    line-height: 1.2 !important;
    margin: 0 !important;
    white-space: nowrap !important; 
    overflow: hidden !important;       
    clear: none !important;         
    float: none !important;
}

.tpm-hours-grid label:hover {
    background: #f0f6fc !important;
}

.tpm-hours-grid input[type="checkbox"] {
    margin: 0 8px 0 0 !important;
    display: inline-block !important;
    flex-shrink: 0 !important;
}

/* Container for the file lists */
.tpm-file-queue {
    list-style: none;
    padding: 0;
    margin: 10px 0 0 0;
}

/* Individual file row */
.tpm-file-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    background: #fff;
    padding: 8px 12px;
    margin-bottom: 6px;
    border: 1px solid #ccd0d4;
    border-radius: 4px;
    color: #2271b1;
    font-size: 0.9em;
    font-weight: 600;
}

/* The 'X' delete button */
.tpm-remove-file {
    color: #d63638;
    cursor: pointer;
    font-size: 18px;
    line-height: 1;
    padding: 2px 5px;
}

.tpm-remove-file:hover {
    background: #f0f0f0;
    border-radius: 3px;
}

.tpm-status-msg p {
    margin: 5px 0 0 0;
    font-size: 0.85em;
    font-weight: bold;
}

/* Submissions Table Styling */
.tpm-row-clickable:hover { 
    background-color: #f0f6fb !important; 
    cursor: pointer; 
}

.status-badge { 
    padding: 3px 8px; 
    border-radius: 3px; 
    font-size: 11px; 
    font-weight: bold; 
    border: 1px solid; 
    display: inline-block;
}

.status-approved { background: #e7f5ea; color: #185a1b; border-color: #c3e6cb; }
.status-pending  { background: #fcf9e8; color: #856404; border-color: #ffeeba; }
.status-rejected { background: #f8d7da; color: #721c24; border-color: #f5c6cb; }

/* Modal Styling */
#tpm-app-modal {
    display: none; 
    position: fixed; 
    z-index: 9999; 
    left: 0; 
    top: 0; 
    width: 100%; 
    height: 100%; 
    background: rgba(0,0,0,0.5);
}

/* Make the modal container a Flexbox column */
.tpm-modal-inner {
    display: flex;
    flex-direction: column;
    position: relative;
    margin: 5% auto;
    width: 60%;          /* A bit wider for readability */
    max-height: 85vh;    /* Prevents it from going off-screen */
    background: #fff;
    border-radius: 8px;
    box-shadow: 0 10px 25px rgba(0,0,0,0.2);
    overflow: hidden;    /* Critical: locks the inner content */
}

/* Now sections are defined by their flex behavior */
#tpm-modal-header {
    flex: 0 0 auto;      /* Header stays fixed size */
    background: #f9f9f9;
    border-bottom: 1px solid #ddd;
    padding: 20px;
}

#tpm-modal-body {
    flex: 1 1 auto;      /* Body takes all remaining space */
    overflow-y: auto;    /* Only this part scrolls */
    padding: 20px;
}

#tpm-modal-footer {
    flex: 0 0 auto;      /* Footer stays fixed at bottom */
    background: #f9f9f9;
    border-top: 1px solid #ddd;
    padding: 15px 20px;
}

/* Give padding back to the individual sections */
.tpm-modal-section {
    padding: 20px;
}

.tpm-modal-section { padding: 15px 0; border-bottom: 1px solid #eee; }
.tpm-modal-section:last-child { border-bottom: none; }

.tpm-modal-header-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; }
.tpm-modal-header-grid div { margin-bottom: 5px; }

.tpm-modal-actions { 
    display: flex; 
    justify-content: space-between; 
    align-items: center; 
    margin-top: 10px; 
}

.tpm-button-group .button { margin-left: 10px; }

#tpm_reject_reason { border: 1px solid #d63638; }

.tpm-details-grid {
    display: grid;
    grid-template-columns: 1fr 1fr; /* Two columns */
    gap: 15px;
}

.tpm-details-grid .full {
    grid-column: span 2; /* Full width items */
    border-top: 1px solid #eee;
    padding-top: 10px;
}

#close-tpm-modal {
    position: absolute;
    left: 10px;
    top: 5px;
    cursor: pointer;
    font-size: 28px;
    color: #666;
    z-index: 100;
}

#close-tpm-modal:hover { color: #000; }

/* Add padding to the header so the title doesn't hit the X */
#tpm-modal-header {
    padding-left: 40px; 
}

.tpm-type-badge {
    padding: 4px 12px;
    border-radius: 20px;
    font-size: 16px;
    font-weight: 600;
}

/* Color coding for the types */
.tpm-type-cancel    { background: #fee2e2; color: #991b1b; } /* Soft Red */
.tpm-type-makeup    { background: #dcfce7; color: #166534; } /* Soft Green */
.tpm-type-fieldtrip { background: #dbeafe; color: #1e40af; } /* Soft Blue */

/* Στυλ για απενεργοποιημένα κουμπιά στο modal */
.tpm-modal-footer button:disabled {
    background-color: #ccc !important;
    border-color: #bbb !important;
    color: #888 !important;
    cursor: not-allowed;
    opacity: 0.7;
}

#tpm_reject_reason[readonly] {
    background-color: #f0f0f1;
    color: #666;
    border-color: #dcdcde;
    cursor: not-allowed;
}