Change the Reference Tooltips Wikipedia gadget (CSS, JS) to use https://www.mediawiki.org/wiki/File:Gear_icon.svg.  You will probably need to crop it.  It must use SVG in supported browsers, but still render (using PNG) in older browsers.

Currently, only a PNG is used.  Change it to the Gear_icon.svg image, and use the technique given at http://pauginer.com/post/36614680636/invisible-gradient-technique / https://git.wikimedia.org/blob/mediawiki%2Fcore.git/ee8f8cc583aa74e7123b47118ecf16a6f627a0cc/resources%2Fsrc%2Fmediawiki.less%2Fmediawiki.mixins.less#L32 (you will not be able to actually use the mixin).

The code files you need to start from are given as the first two links of this description (CSS, JS).

You are not required to actually edit those pages or submit your work on Gerrit (since this is outside the scope of Gerrit).  You can test your code as a user script (login then visit https://en.wikipedia.org/wiki/Special:MyPage/common.js and https://en.wikipedia.org/wiki/Special:MyPage/common.css); you can also submit it by linking to these pages (with Special:MyPage replaced with User:YourUserName).

If you need to upload images (e.g. a resized or cropped SVG and/or PNG), you can use http://commons.wikimedia.org/wiki/Special:UploadWizard .  Be sure to cite the original image (Gear_icon.svg) you based your work on and copy the license information.

Please read https://bugzilla.wikimedia.org/show_bug.cgi?id=50843 before starting.

Students are required to read Wikimedia's general instructions first.