Add --color-logo for text that should match logo color (go-gitea#34639)

Add a new color that indicates the logo's primary color and use it in
the frontpage over previous green color. This will be useful for
customization.

<img width="1347" alt="Screenshot 2025-06-07 at 16 53 34"
src="https://github.com/user-attachments/assets/496aa81f-c910-4c28-bd12-f2473a68bbab"
/>

Author: silverwind

web_src/css/home.css

@@ -21,7 +21,7 @@
 }
 
 .home .hero .svg {
-  color: var(--color-green);
+  color: var(--color-logo);
   height: 40px;
   width: 50px;
   vertical-align: bottom;
@@ -40,7 +40,7 @@
 }
 
 .home a {
-  color: var(--color-green);
+  color: var(--color-logo);
 }
 
 .page-footer {

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

@@ -185,6 +185,7 @@ gitea-theme-meta-info {
   --color-orange-badge-bg: #f2711c1a;
   --color-orange-badge-hover-bg: #f2711c4d;
   --color-git: #f05133;
+  --color-logo: #609926;
   /* target-based colors */
   --color-body: #1b1f23;
   --color-box-header: #1a1d1f;

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

@@ -185,6 +185,7 @@ gitea-theme-meta-info {
   --color-orange-badge-bg: #f2711c1a;
   --color-orange-badge-hover-bg: #f2711c4d;
   --color-git: #f05133;
+  --color-logo: #609926;
   /* target-based colors */
   --color-body: #ffffff;
   --color-box-header: #f1f3f5;