Fix various CSS issues (#13769)

- Fix black text being white on base theme
- Fix file/blame button group
- Fix label margin in dropdown (regression from graph pr)

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/_base.less

@@ -628,7 +628,7 @@ a.muted:hover,
     }
 
     &.black {
-      color: var(--color-body);
+      color: var(--color-text);
 
       &:hover {
         color: #000000;
@@ -1433,6 +1433,19 @@ a.ui.label:hover {
   color: var(--color-text);
 }
 
+.ui.button:hover {
+  background: var(--color-hover);
+  color: var(--color-text);
+}
+
+.ui.buttons .button:first-child {
+  border-left: 1px solid var(--color-secondary);
+}
+
+.ui.buttons .button + .button {
+  border-left: none;
+}
+
 .ui.blue.button,
 .ui.blue.buttons .button,
 .ui.primary.button,
@@ -1617,10 +1630,6 @@ table th[data-sortt-desc] {
   }
 }
 
-.dropdown .ui.label {
-  margin-left: 0 !important;
-}
-
 .ui.dropdown .menu .item {
   border-radius: 0;
 }