Backport #25634 

Resolves https://github.com/go-gitea/gitea/issues/25622

<details>
<summary>Screenshots</summary>

![Bildschirmfoto vom 2023-07-02

20-47-34](https://github.com/go-gitea/gitea/assets/47871822/a8a0bff6-9ae3-48f3-b008-00c196a3f8fd)
![Bildschirmfoto vom 2023-07-02

20-47-45](https://github.com/go-gitea/gitea/assets/47871822/172a0021-af74-4690-aa67-0e66688ce733)
![Bildschirmfoto vom 2023-07-02

20-48-37](https://github.com/go-gitea/gitea/assets/47871822/14572ebd-0106-4c8a-ba27-b6b631375ee6)
![Bildschirmfoto vom 2023-07-02

20-49-08](https://github.com/go-gitea/gitea/assets/47871822/7c0ba3aa-1712-482c-aae9-13394dbdaf8a)
![Bildschirmfoto vom 2023-07-02

20-50-28](https://github.com/go-gitea/gitea/assets/47871822/8bd68e26-099a-4abd-8817-16d52af13167)
![Bildschirmfoto vom 2023-07-02

20-51-46](https://github.com/go-gitea/gitea/assets/47871822/3beab8c6-3747-4829-be50-bafaed11000c)
![Bildschirmfoto vom 2023-07-02

20-54-12](https://github.com/go-gitea/gitea/assets/47871822/51f82ef3-a32c-4c27-9056-e8711ed469cc)

</details>

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

We were missing overrides for `:focus` and `:active` styles which I've
added here along with two new color variants `dark-1` and `dark-2` for
them. Fomantic UI has 4 different colors but I think 3 are sufficient. I
also changed it on arc-green so button goes darker when pressed.

<img width="129" alt="Screenshot 2023-05-04 at 01 21 43"
src="https://user-images.githubusercontent.com/115237/236072060-7389276a-275b-4d3e-aa52-20b37c6e6d92.png">
<img width="130" alt="Screenshot 2023-05-04 at 01 17 59"
src="https://user-images.githubusercontent.com/115237/236071818-0e46414a-33db-4bb2-a3bd-35b514a8a2d0.png">
<img width="129" alt="Screenshot 2023-05-04 at 01 18 07"
src="https://user-images.githubusercontent.com/115237/236071819-562b1e38-541f-432b-b3b6-48e6d7594d00.png">
<img width="131" alt="Screenshot 2023-05-04 at 01 18 13"
src="https://user-images.githubusercontent.com/115237/236071820-89b7dba9-ce6c-48e5-a075-9053063e6ad3.png">
<img width="133" alt="Screenshot 2023-05-04 at 01 18 30"
src="https://user-images.githubusercontent.com/115237/236071823-b6fe2df4-b3f0-4dc8-97a8-f90ba6d19bec.png">
<img width="133" alt="Screenshot 2023-05-04 at 01 18 40"
src="https://user-images.githubusercontent.com/115237/236071824-b02ce61a-2367-4c29-8a25-45f231f5e5ee.png">

One misc change includes some fixes to editor and slightly darker
selection.

<img width="1245" alt="Screenshot 2023-05-28 at 19 16 19"
src="https://github.com/go-gitea/gitea/assets/115237/1ea4a4b6-26ba-45af-9cbc-5b8c476c2338">

There was some recent discussion about this in Discord `ui-design`
channel and the conclusion was that
https://github.com/go-gitea/gitea/issues/24305 should have fixed their
OS font installation to have semibold weights.

I have now tested this 601 weight on a Windows 10 machine on Firefox
myself, and I immediately noticed that bold was excessivly bold and
rendering as 700 because browsers are biased towards bolder fonts. So
revert this back to the previous value.

Reorganize various CSS files for clarity, group together by subdirectory
in `index.css`. This reorders some of the rules, but I don't think it
should introduce any issues because of that.

Since 2015/2016, there is a global pollution: ".ui.left" / ".ui.right".

Fomantic UI doesn't work this way, it just conflicts with many Fomantic
definitions.

This PR starts the cleaning work of such techinical debts.

And, the "label list" page has been quite messy for long time, for
example, why "li" appears in "div" ......

And fix #24296

<details>


![image](https://user-images.githubusercontent.com/2114189/235051281-54c5374c-b5fd-4b5f-9aa2-02d4bb2d9112.png)


![image](https://user-images.githubusercontent.com/2114189/235055703-2ba042e0-4db7-4e63-8646-02f390d496b5.png)


![image](https://user-images.githubusercontent.com/2114189/235056310-4f6ffdc2-5758-4927-8fb8-314d9fb72a6b.png)


![image](https://user-images.githubusercontent.com/2114189/235058400-dab1c9ec-3325-4671-8345-aee6b0b68042.png)


![image](https://user-images.githubusercontent.com/2114189/235058424-85509532-b9bc-43ad-b00f-a87184c60f22.png)

</details>