Tasks icon CSS not compatible with Gin theme

I realized today that my last round of changes don't work very well with Gin's dark mode. I decided to embrace Gin more fully and use its color variables since they're designed to work well in both light and dark modes. Screenshots of the results below.

Given that Moderation Sidebar's status is "Maintenance fixes only" and that the maintainers might not want to add all this for Gin while it's a moving target (if ever), I created Gin Moderation Sidebar → . It adds a "Gin-style" icon, has light/dark-mode friendly Gin-based colors, fixes the Moderation Sidebar tab placement (far right), and works with all Gin toolbar modes.

Since the gin moderation sidebar module is a bit more than this original patch... gonna say that people should just use that submodule instead.