Remove CSS `has` selector and improve various styles (#26891)

Remove CSS `has` selector and improve various styles (#26891)

Replace  #26850

Major changes:

1. Remove all `has` selectors, it is still not supported by firefox.
Actually there could be some more general and clearer approaches
2. Remove `two-toggle-buttons`, the `.ui.buttons` just works well
3. Rewrite the `.ui.buttons` border styles, see the screenshots
4. Remove the "fine-tuning" paddings from the the flex children, they
could layout themselves well.

![image](https://github.com/go-gitea/gitea/assets/2114189/a32ed6f3-60f7-43d5-9492-62c45d2397f6)

![image](https://github.com/go-gitea/gitea/assets/2114189/5cb173c5-c942-4237-8cb4-2697220b3f06)

![image](https://github.com/go-gitea/gitea/assets/2114189/8a1c12b3-a632-48ff-b1a7-a01a4417f821)

![image](https://github.com/go-gitea/gitea/assets/2114189/46bde1bd-9113-4231-965d-6ec9076f6a3b)