doc: apply logical ordering to CSS variables

The CSS variables are not in any particular logical order as far as I
can tell, with certain items that would seem to go together not grouped.
This puts them in alphabetical order.

PR-URL: #39169
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>

doc/api_assets/style.css

@@ -25,23 +25,23 @@
   --gray7: #c1c1c1;
   --grey8: #ddd;
 
+  --api-stability-links-bg: rgba(255, 255, 255, .4);
   --color-brand-primary: var(--gray6);
   --color-brand-secondary: var(--green1);
-  --color-text-primary: var(--gray6);
-  --color-text-secondary: var(--green2);
   --color-fill-app: var(--white);
+  --color-fill-side-nav: var(--gray6);
+  --color-links: var(--green1);
   --color-text-nav: var(--gray3);
+  --color-text-primary: var(--gray6);
+  --color-text-secondary: var(--green2);
   --highlight-background-color: var(--white-smoke);
-  --color-links: var(--green1);
-  --color-fill-side-nav: var(--gray6);
-  --api-stability-links-bg:  rgba(255, 255, 255, .4)
 }
 
 .dark-mode {
-    --color-links: var(--green5);
     --color-fill-app: var(--black1);
-    --color-text-primary: var(--white);
     --color-fill-side-nav: var(--black3);
+    --color-links: var(--green5);
+    --color-text-primary: var(--white);
     --highlight-background-color: var(--black2);
 }