[Feature] Better plugin CSS injection

[shdwmtr]

Before Requesting

• I found no existing issue matching my feature request
• This request is related to Millennium, and not a request for a specific theme/addon

Describe the feature you'd like!

A better way to inject CSS into windows without having to manage CSS files on disk

Anything else?

POC:

import styles from './styles.css';
Millennium.insertStyles(document /* the window to inject into */, styles);

Here we statically rollup CSS into the dist JavaScript instead of leaving it on disk, and having to move it into steamui during runtime.