Project board improvements (#15429)

* Project board improvements

- Fix link colors
- Extract CSS to own file
- Various minor tweaks to make it look better

Fixes: https://github.com/go-gitea/gitea/issues/15424
Fixes: https://github.com/go-gitea/gitea/issues/15506
Fixes: https://github.com/go-gitea/gitea/pull/15511

* fix squashed cards on small view area

* more css fixes, add second row from issue list

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/themes/theme-arc-green.less

@@ -107,6 +107,7 @@
   --color-expand-button: #3c404d;
   --color-placeholder-text: #6a737d;
   --color-editor-line-highlight: var(--color-primary-light-5);
+  --color-project-board-bg: var(--color-secondary-light-2);
 }
 
 .ui.horizontal.segments > .segment {
@@ -690,10 +691,6 @@ footer .container .links > * {
   border-color: #383c4a;
 }
 
-.board-column {
-  background-color: rgba(0, 0, 0, .2) !important;
-}
-
 .tribute-container {
   box-shadow: 0 .25rem .5rem rgba(0, 0, 0, .6);
 }