style: fix poor visibility of some text

[WofWca]

I searched for (almost) all explicitly defined colors
and fixed the most obvious.

[r10s]

did not test, just a general question:

is it a usual, modern way to define grey tones using opacity instead of a color? i would expect that to be slower, but apart from that, also not that much easier to read - and that may also be unexpected, when it comes to changed backgrounds - or to child items

EDIT: wrt slowness i found https://stackoverflow.com/questions/38523826/text-css-rendering-performance-rgba-vs-hex-vs-opacity - might not be that relevant, but still, things sum up :)

[WofWca]

is it a usual, modern way to define grey tones using opacity instead of a color?

IDK whether you can call it a standard, I think there are not that many people who care about CSS XD
I'm just trying to save up on defining extra variables (which I'm not sure if that's something that needs to be done).

[Simon-Laux]

I recently discussed css variables with @nicodh, basically we want to come up with some rules, that we also document in the docs/STYLES.md.

The idea is outlined in #3018
Basically:

• never hardcode colors, use css vars instead
• every component gets a css var defined in theme base
• in theme base the colors get hardcoded or is applicable set dynamically by deriving them from the sass variables defined in the themes.