I was this years old to know aside of HTML, you can also convert CSS file to PHP in order to use some PHP shenanigans. Just add a header to indicate that it’s of type text/css:

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<?php header("Content-Type: text/css"); ?>
@import url('../styles.css?v=<?=filemtime('../styles.css')?>');

.category-header {
    background-color: var(--color-bg-alt);
    backdrop-filter: blur(5px);
    ...
}

and inside the HTML file you can link the stylesheet like usual, except changing the .css to .php

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<link rel="stylesheet" href="gallery.php" />

Wild.