Branch page and misc css improvements (#15208)

- Improve branches page, increase icon size, use octicons, use css vars
- Style placeholder color via css var
- Slightly increase contrast of input fields and active/hover states
- Add styling for select boxes in arc-green

web_src/less/_repository.less

@@ -1261,7 +1261,7 @@
         max-width: 90px;
 
         &:last-child {
-          border-left: 1px solid var(--color-secondary);
+          border-left: 1px solid var(--color-secondary-dark-2);
         }
       }
 
@@ -1280,7 +1280,7 @@
       .bar {
         height: 4px;
         position: absolute;
-        background-color: var(--color-secondary);
+        background-color: var(--color-secondary-dark-2);
 
         &.bar-behind {
           right: 0;