Defines new css colours for theming:
```css
--color-selection-bg: var(--color-primary-light-1);
--color-selection-fg: var(--color-white);
```
which are then used both in the `base.css` and in the relative-time shadow object.
This, is how it looks for me when selecting before and after this patch (my Browser’s accent colour used for selection is orange):
| | Before | After |
|---|---|---|
| Forgejo Light |  |  |
| Forgejo Dark | |  |
| Gitea Light |  |  |
| Gitea Dark |  |  |
Co-authored-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11231
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: Robert Wolff <mahlzahn@posteo.de>
Co-committed-by: Robert Wolff <mahlzahn@posteo.de>
