Rename CSS variables and improve colorblind themes (#36353)

Followup https://github.com/go-gitea/gitea/pull/36215, rename the
variables for consistency with existing vars and change green to value
of `--color-blue` in the relevant color blind themes:

<img width="1305" height="303" alt="image"
src="https://github.com/user-attachments/assets/3d131ab7-99ab-4b03-93ab-715ce0030b08"
/>


The blue coloring also matched GitHub:

<img width="1313" height="393" alt="image"
src="https://github.com/user-attachments/assets/f97e35b2-4ff4-49b0-841f-ffd49a02e03d"
/>

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

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

@@ -1,4 +1,4 @@
-@import "./theme-gitea-dark-protanopia-deuteranopia.css";
+@import "./theme-gitea-dark.css";
 
 gitea-theme-meta-info {
   --theme-display-name: "Dark";
@@ -7,9 +7,9 @@ gitea-theme-meta-info {
 }
 
 /* blue/yellow colorblind-friendly colors */
-/* from GitHub: blue yellow blindness is based on red green blindness, and --diffBlob-deletion-* restored to the normal theme color */
 :root {
-  --color-diff-removed-linenum-bg: #482121;
-  --color-diff-removed-row-bg: #301e1e;
-  --color-diff-removed-word-bg: #6f3333;
+  --color-diff-added-fg: #58a6ff;
+  --color-diff-added-linenum-bg: #243d5d;
+  --color-diff-added-row-bg: #132339;
+  --color-diff-added-word-bg: #214d87;
 }