Simplify `Flat` shader. Include `Background` docs page.

[sirdoombox]

Changes

• Simplifed the Flat shader, there was no need for the iDark branch nor the * 1 assignment. The underlying iBase colour handles everything for us.
  • This should actually marginally improve performance as GPUs hate branching.

Docs

• Included a Background page in theming to walk people through the basics of changing the background style and how custom shaders work.
  • Also includes a warning about animations.

[kikipoulet]

Awesome ! 😳