We have very few core YUI modules with their own CSS in Moodle, so you'll have to fake some first: Prep Open lib/yui/src/tooltip/meta/tooltip.json add "moodle-core-tooltip-skin" to the list of requires create the directory structure lib/yui/src/tooltip/assets/skins/sam/ Add some recognisable CSS content to a file in that directory named moodle-core-tooltip.css run Shifter on the module/whole of Moodle Now to test Navigate to the Login page Take a look at the list of stylesheets in the network panel of your dev toolkit look for an attempt to load a skin for moodle-core-tooltip view that files content Confirm that your recognisable content was found And just to make sure: Open http://tangerine.local/moodle/theme/yui_combo.php?moodle/-1/core/tooltip/tooltip.css (where tangerine.local/moodle is the path to your testing moodle installation) confirm that your content is shown