Sidebar auto-generated color palette doesn't revert to browser theme when unset

### Preliminary Checks - [x] I have read and understood the important section above. - [x] I have searched existing issues and avoided creating duplicates. - [x] I am not filing an enhancement request. - [x] I have checked that this issue cannot be reproduced on Mozilla Firefox. - [x] I have checked that this issue can be reproduced once I removed all my Mods and Custom CSS. ### What happened? The sidebar theming tool (which is pretty neat!) is overwriting custom theming in all situations. It makes sense to overwrite when chosen. When no choice is made, it would make more sense to use browser css-variables to keep it themed correctly. At the moment, it appears to be stuck in light-mode. ### Expected behavior Sidebar reflects system browser theming when unset ### Actual behavior Stuck in light mode ### Steps to reproduce Install browser, update css files for theming, not able to update sidebar ### Screenshots and videos _No response_ ### Version 1.21.8b (Firefox 152.0.6) (aarch64 ### What platform are you seeing the problem on? Linux (Tarball) ### What component is this issue related to? Tab Folders ### Relevant log output if applicable ```shell ```