[THEME] Revert darker tone on labels

- In #1782 I made these darker to make some elements look good, but this
variable is used quite broadly which causes some issues. So reverting
the darker tone to the more light tone (which is identical with what's
the behavior before #1782 was merged).
- Resolves https://codeberg.org/forgejo/forgejo/issues/2879

web_src/css/themes/theme-forgejo-dark.css

@@ -226,9 +226,9 @@
   --color-nav-bg: var(--steel-900);
   --color-nav-hover-bg: var(--steel-600);
   --color-label-text: #fff;
-  --color-label-bg: #393939;
-  --color-label-hover-bg: #5f5f5f;
-  --color-label-active-bg: #4c4c4c;
+  --color-label-bg: #cacaca5b;
+  --color-label-hover-bg: #cacacaa0;
+  --color-label-active-bg: #cacacaff;
   --color-accent: var(--color-primary-light-1);
   --color-small-accent: var(--color-primary-light-5);
   --color-active-line: var(--color-primary-alpha-20);