Change image transparency grid to CSS (#36711)

These new colors work much better on dark theme than before (where it
was far too bright).

<img width="731" height="533" alt="image"
src="https://github.com/user-attachments/assets/e2979935-87ac-4d0e-80e1-67fe6cd2d6c7"
/>

<img width="736" height="543" alt="image"
src="https://github.com/user-attachments/assets/96da1292-cc77-49bf-aa51-d48b6c7cf2b4"
/>

---------

Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

web_src/css/base.css

@@ -42,7 +42,7 @@
   --gap-inline: 0.25rem; /* gap for inline texts and elements, for example: the spaces for sentence with labels, button text, etc */
   --gap-block: 0.5rem; /* gap for element blocks, for example: spaces between buttons, menu image & title, header icon & title etc */
 
-  --background-view-image: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKCAYAAACNMs+9AAAAG0lEQVQYlWN4+vTpf3SMDTAMBYXYBLFpHgoKAeiOf0SGE9kbAAAAAElFTkSuQmCC") right bottom var(--color-primary-light-7);
+  --background-view-image: repeating-conic-gradient(var(--color-transparency-grid-dark) 0 25%, var(--color-transparency-grid-light) 0 50%) 0 0 / 18px 18px;
 }
 
 @media (min-width: 768px) and (max-width: 1200px) {

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

@@ -247,6 +247,8 @@ gitea-theme-meta-info {
   --color-highlight-bg: #352c1c;
   --color-overlay-backdrop: #080808c0;
   --color-danger: var(--color-red);
+  --color-transparency-grid-light: #2a2a2a;
+  --color-transparency-grid-dark: #1a1a1a;
   accent-color: var(--color-accent);
   color-scheme: dark;
 }

web_src/css/themes/theme-gitea-light.css

@@ -247,6 +247,8 @@ gitea-theme-meta-info {
   --color-highlight-bg: #fffbdd;
   --color-overlay-backdrop: #080808c0;
   --color-danger: var(--color-red);
+  --color-transparency-grid-light: #fafafa;
+  --color-transparency-grid-dark: #e2e2e2;
   accent-color: var(--color-accent);
   color-scheme: light;
 }