Simplify text color selectors and tweak arc-green colors (#21784)

Move the text color rules out of the unneeded `.ui` block, add missing
colors, tweak colors on arc-green to be more readable (red was
particulary bad to read).

Also, this removes the previous inheritance of link colors. I think
links should always be in primary color and if they are to be
discolored, the color should be set on them explicitely.

<img width="165" alt="Screenshot 2022-11-12 at 13 28 30"
src="https://user-images.githubusercontent.com/115237/201474098-700d9fed-3133-43c7-b57e-d4cc5c2795cb.png">

<img width="152" alt="Screenshot 2022-11-12 at 13 18 48"
src="https://user-images.githubusercontent.com/115237/201474156-b6de4cb5-bce8-4553-b3d4-8365aff9a3a7.png">

HTML to test with:

```html
<div class="text red">some text with <a href="#foo">a link</a>.</div>
<div class="text orange">some text with <a href="#foo">a link</a>.</div>
<div class="text yellow">some text with <a href="#foo">a link</a>.</div>
<div class="text olive">some text with <a href="#foo">a link</a>.</div>
<div class="text green">some text with <a href="#foo">a link</a>.</div>
<div class="text teal">some text with <a href="#foo">a link</a>.</div>
<div class="text blue">some text with <a href="#foo">a link</a>.</div>
<div class="text violet">some text with <a href="#foo">a link</a>.</div>
<div class="text purple">some text with <a href="#foo">a link</a>.</div>
<div class="text pink">some text with <a href="#foo">a link</a>.</div>
<div class="text brown">some text with <a href="#foo">a link</a>.</div>
<div class="text grey">some text with <a href="#foo">a link</a>.</div>

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