Fix various legacy problems (#37092)

1.  Fix #36439
2. Fix #37089
3. Fix incorrect layout of admin auth oidc page
4. Fix #35866
5. Fix #35800
6. Fix #36243

web_src/css/markup/content.css

@@ -290,7 +290,9 @@ In markup content, we always use bottom margin for all elements */
    We decide which to show via `data-gitea-theme-dark` on `<html>`, which is
    mirrored from `--is-dark-theme` in JS (so it also works with auto/custom themes).
 */
+html[data-gitea-theme-dark="true"] .markup a[href*="#gh-light-mode-only"],
 html[data-gitea-theme-dark="true"] .markup img[src*="#gh-light-mode-only"],
+html[data-gitea-theme-dark="false"] .markup a[href*="#gh-dark-mode-only"],
 html[data-gitea-theme-dark="false"] .markup img[src*="#gh-dark-mode-only"] {
   display: none;
 }