Use Selection color from design token

[Wumpf]

I set out to apply various other design tokens, but after having a closer look at how not all our design sketches use the exact same colors and the existing struggle in design_tokens.rs, I did only this one impactful one

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I've added a line to CHANGELOG.md (if this is a big enough change to warrant it)

[emilk]

Is it just me, or is that a lot uglier? 😬

[Wumpf]

fits better into the dark design I believe, but I might be just biased by being used to the other color, so this looks new and shiny to me
no strong opinion either way 🤷

[nikolausWest]

I think I like this color better

[emilk]

let's try it then