Without knowing your exact setup, this is hard to say, but it looks like some code is adding styling that overrules some parts of your theme’s styles. You could try adding !important to that font size and see what happens.
Otherwise, you could copy that line that works in the inspector into your theme’s functions.php like this:
add_action('admin_head', 'wpse426010_custom_admin_styles', 999);
function wpse426010_custom_admin_styles() {
echo '<style>
... your style here
</style>';
}
Note the high priority of the function to make sure this style is added after everything else.
Beware that this hack will be specific to your current setup. Because it is likely the unintended consequence of some code, it could stop working when that code (Mai Delight, Genesis or some plugin) is updated.