Load CSS later on wp_head

simple-css loads its custom styles at wp_head:10:

add_action( 'wp_head', 'simple_css_generate' );

(10 is the default priority.)

This causes race conditions when plugins or themes load inline styles with the same priority, so that sometimes the custom styles override, and sometimes they don’t. I saw this issue specifically with the Arras theme, but there are lots of plugins that load stuff at wp_head, so I’m sure it’s a more common issue.

Anyway, I’d suggest that simple-css styles should always be among the last to load. For my client site, I’ve done the following:

add_action( 'plugins_loaded', function() {
    remove_action( 'wp_head', 'simple_css_generate' );
    add_action( 'wp_head', 'simple_css_generate', 999999 );
} );

For the plugin, I would suggest moving to a higher priority:

add_action( 'wp_head', 'simple_css_generate', 999 );