feat: CSS3 variables & improved CSS dev experience

This PR is based off the original work done in https://github.com/unraid/webgui/pull/1425. Huge shout out to @BaileyJM02 for the hard work with the initial setup for this.

The purpose of this PR is to lay the ground work for future CSS enhancements and improving the developer experience for styles.

Removing the idea of have multiple CSS files for theming differences. Ideally there's a singular base CSS file (should work for both layout types) with all colors using CSS variables. And the theme files are used to solely set CSS variables for color differences, instead of copied and pasted styles with different colors. Less duplicated code should make maintaining and adding new styles easier while also being less prone to errors.