Your changes are not applied because the selector in the categories-list.scss file is more specific. If you want your styles to apply, you need to change your selector so that it either has the same specificity or higher.
Also as a side note, if you want to use the font-size variables, you don’t actually need to add all of these to your theme:
$font-up-6: 2.296em;
$font-up-5: 2em;
$font-up-4: 1.7511em;
$font-up-3: 1.5157em;
$font-up-2: 1.3195em;
$font-up-1: 1.1487em; // 2^(1/5)
$font-0: 1em;
$font-down-1: 0.8706em; // 2^(-1/5)
$font-down-2: 0.7579em; // Smallest size we use based on the 1em base
$font-down-3: 0.6599em;
$font-down-4: 0.5745em;
$font-down-5: 0.5em;
$font-down-6: 0.4355em;
// Common line-heights
$line-height-small: 1;
$line-height-medium: 1.2; // Headings or large text
$line-height-large: 1.4; // Normal or small text
You only need to add this:
@import "common/foundation/variables";
and all the variables (including font-size) should be available for you to use.