Do not use thin scrollbars on Firefox (#16738)

In #7269, thin scrollbars were added in Arc Green theme. It got moved
in base theme in #13361.

This PR removes the use of thin scrollbars which causes an
accessibility issue. The scrollbars become too thin to be dragged.

Signed-off-by: Elouan Martinet <exa@elou.world>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_base.less

@@ -175,7 +175,6 @@ details summary > * {
 }
 
 * {
-  scrollbar-width: thin;
   scrollbar-color: var(--color-primary) transparent;
   caret-color: var(--color-caret);
 }