Fix some broken css (#23560)

1. The "close" inside "modal" are likely broken for long time
    * There is no var called `--body-color`
    * There is no `fullscreen modal`
* The `.ui.modal > .close.inside` doesn't seem to match most icons. It
only matches a few like "fork-repo-modal" or "adopt repo". Other places
are just buggy code copied again and again.
2. Convert the legacy `&:hover` LESS syntax to CSS syntax

web_src/css/base.css

@@ -1173,10 +1173,8 @@ a.ui.card:hover,
   border-color: var(--color-secondary);
 }
 
-.ui.modal > .close.inside,
-.ui.fullscreen.modal > .close {
-  top: 11px; /* align modal close icon, for example admin notices */
-  color: var(--body-color);
+.ui.modal > .close.inside {
+  color: var(--color-text);
 }
 
 .ui.basic.table > tbody > tr {

web_src/css/code/linebutton.css

@@ -17,8 +17,8 @@
   left: 0;
   transform: translateX(-70%);
   cursor: pointer;
+}
 
-  &:hover {
-    color: var(--color-primary);
-  }
+.code-line-button:hover {
+  color: var(--color-primary);
 }