Implement userspace CSS to allow further customization of the UI

[IzKuipers]

Testing of this has already been performed: Making a markdown file on ArcFS and giving it a style tag already applies user styling to ArcOS as long as the Markdown file is rendered using Markdown Viewer:

<style>
  * {
    color: #f0f;
  }
</style>

The effects of this styling is reverted when Markdown Viewer is closed. We can apply the same principle to a style tag that only renders when the desktop is loaded, that way other states aren't affected.

[NikNikovsky]

How's that going

[IzKuipers]

I'll start development of this first thing in the morning, I have some ideas

[NikNikovsky]

Nice