Rework checkbox styling, remove input border hover effect (#36870)

- Rework all checkbox styling to be consistent inside and outside
markup.
- Remove `input` border hover effect. Was too subtle and honestly
unneeded, consistent with GitHub.
- Increase `input` border contrast slightly.
- Some small spacing fixes in Markup (nested tasklist and spacing after
checkbox).

<img width="221" height="222" alt="Screenshot 2026-03-09 at 08 18 19"
src="https://github.com/user-attachments/assets/9e66abee-7102-4abe-9b00-e3f9b24ed735"
/>
<img width="226" height="217" alt="Screenshot 2026-03-09 at 08 18 10"
src="https://github.com/user-attachments/assets/33cdac26-4479-41da-9488-e60d70c5c997"
/>
<img width="79" height="218" alt="Screenshot 2026-03-09 at 08 17 32"
src="https://github.com/user-attachments/assets/ae1064a2-2bb3-44e7-a00b-2f4f5aad4241"
/>
<img width="267" height="297" alt="Screenshot 2026-03-09 at 08 17 07"
src="https://github.com/user-attachments/assets/1237fa98-0d94-4023-a87d-190d89c57421"
/>
<img width="558" height="260" alt="Screenshot 2026-03-09 at 08 21 04"
src="https://github.com/user-attachments/assets/1908a794-3394-494c-b2d5-470c00c668d1"
/>

---------

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/base.css

@@ -15,8 +15,8 @@
   /* line-height: use the default value as "modules/normalize.css" */
   --line-height-default: normal;
   /* images */
-  --checkbox-mask-checked: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="-1 -1 18 18" width="16" height="16"><path fill-rule="evenodd" d="M13.78 4.22a.75.75 0 010 1.06l-7.25 7.25a.75.75 0 01-1.06 0L2.22 9.28a.75.75 0 011.06-1.06L6 10.94l6.72-6.72a.75.75 0 011.06 0z"></path></svg>');
-  --checkbox-mask-indeterminate: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16"><path fill-rule="evenodd" d="M2 7.75A.75.75 0 012.75 7h10a.75.75 0 010 1.5h-10A.75.75 0 012 7.75z"></path></svg>');
+  --checkbox-mask-checked: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="12" height="9" viewBox="0 0 12 9"><path fill-rule="evenodd" d="M11.78.22a.75.75 0 0 1 0 1.061L4.52 8.541a.75.75 0 0 1-1.062 0L.202 5.285a.75.75 0 0 1 1.061-1.061l2.725 2.723L10.718.22a.75.75 0 0 1 1.062 0"/></svg>');
+  --checkbox-mask-indeterminate: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="10" height="2" viewBox="0 0 10 2"><path fill-rule="evenodd" d="M0 1a1 1 0 0 1 1-1h8a1 1 0 1 1 0 2H1a1 1 0 0 1-1-1" clip-rule="evenodd"/></svg>');
   --octicon-chevron-right: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16"><path d="M6.22 3.22a.75.75 0 0 1 1.06 0l4.25 4.25a.75.75 0 0 1 0 1.06l-4.25 4.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042L9.94 8 6.22 4.28a.75.75 0 0 1 0-1.06Z"></path></svg>');
   --select-arrows: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16"><path d="m4.074 9.427 3.396 3.396a.25.25 0 0 0 .354 0l3.396-3.396A.25.25 0 0 0 11.043 9H4.251a.25.25 0 0 0-.177.427m0-1.957L7.47 4.073a.25.25 0 0 1 .354 0L11.22 7.47a.25.25 0 0 1-.177.426H4.251a.25.25 0 0 1-.177-.426"/></svg>');
   /* other variables */
@@ -27,7 +27,7 @@
   --height-loading: 16rem;
   --min-height-textarea: 132px; /* padding + 6 lines + border = calc(1.57142em + 6lh + 2px), but lh is not fully supported */
   --tab-size: 4;
-  --checkbox-size: 15px; /* height and width of checkbox and radio inputs */
+  --checkbox-size: 14px; /* height and width of checkbox and radio inputs */
   --page-spacing: 16px; /* space between page elements */
   --page-margin-x: 32px; /* minimum space on left and right side of page */
   --page-space-bottom: 64px; /* space between last page element and footer */