WP filter to alter admin CSS styles?

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You can use the wp_add_inline_style() function to override styles in an already-enqueued stylesheet:

add_action( 'admin_enqueue_scripts', 'wpse127850_css_overrides' );
function wpse127850_css_overrides() {    
     wp_add_inline_style( 'color', '.mycolor { background: #ccc; }' );
}

You’ll need to find the handle of the stylesheet you want to affect — I’ve guessed at color but there’s no guarantee I’m right.

References

Edited to answer questions from the comments

  1. The function name can be whatever you want — it just has to match the 2nd parameter of the add_action() call.

  2. color is the CSS file’s “handle” — if you View Source of your admin page, you’ll see that all the stylesheets are called like this:

    <link rel="stylesheet" 
 id='the-erudite-css' 
 href="http://example.com/wp-content/themes/the-erudite/css/erudite.css?ver=3.8"
 type="text/css" media="all" />

    The id part contains the CSS file’s handle, followed by -css. So in my example, the handle is the-erudite.

  3. I know this because I went looking for this ability one day, and figured it out by a combination of Google and Codex searching. The WordPress Codex is a fantastic resource for any WordPress development you’re trying to do. If you haven’t yet, you should visit — and probably bookmark — it.