Summary: The Basic, Red, and Green "Simple Button" on http://phorge.localhost/uiexample/view/PHUIButtonExample/ have their separate "Base" colors defined. The Yellow "Simple Button" instead takes its colors from "Shade Yellow" and thus shares them. That makes skinning harder and more surprising when you try to have a usable Dark Mode (fixing Dark Mode is a separate patch though). Refs T15056 Test Plan: * Look at the four colors in `phui-button-simple.css`, compare variables, and dramatically raise an eyebrow * Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Default (Use Standard Colors)` * Look at Yellow Buttons. They are shy but some sometimes they graze on http://phorge.localhost/uiexample/view/PHUIButtonExample/ so watch out! Reviewers: O1 Blessed Committers, mainframe98 Reviewed By: O1 Blessed Committers, mainframe98 Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T15056 Differential Revision: https://we.phorge.it/D26374