Additions to developer theme docs

[TheCommieAxolotl]

• Adds a note about using backdrop-filter with window transparency.
• Notes the existence of --os-accent-color.
• Demonstrates overriding custom properties.

Also:

• text-wrap: pretty on body text

[TheCommieAxolotl]

Also - I noticed that .vitepress/config.ts:

docs/.vitepress/config.ts

Line 145 in 96472ca

theme: "dark-plus",

limits the shiki theme to only dark-plus.

I have amended this in a separate commit which can be merged into this PR if appropriate:
TheCommieAxolotl@2a2c0ba

[zerebos]

Feel free to add the Shiki theme commit tho I have been debating dropping light support entirely.

Not sure how I feel about pretty text wrap as it's chrome only so it would cause minor inconsistencies across browsers. Are you certain that selector covers all cases?