Fix color of tertiary button on dark theme (go-gitea#22739)

Before:
<img width="266" alt="Screenshot 2023-02-03 at 14 07 34"
src="https://user-images.githubusercontent.com/115237/216611151-92e98305-c4b5-42f3-b2e2-8b1b805fa644.png">

After:
<img width="271" alt="Screenshot 2023-02-03 at 14 07 52"
src="https://user-images.githubusercontent.com/115237/216611156-878a8a75-39a1-415b-9b6d-4f035985444e.png">

This is the only instance of such a button in all templates.

---------

Co-authored-by: Lauris BH <[email protected]>
Co-authored-by: Lunny Xiao <[email protected]>

Author: 3 people

web_src/less/_base.less

@@ -2126,9 +2126,7 @@ a.ui.label:hover {
   border: 1px solid var(--color-light-border);
   color: var(--color-text);
 }
-.ui.tertiary.button {
-  border: none;
-}
+
 .page-content .ui.button {
   box-shadow: none !important;
 }
@@ -2244,6 +2242,15 @@ a.ui.label:hover {
   color: var(--color-secondary-dark-8) !important;
 }
 
+.ui.tertiary.button {
+  color: var(--color-text-light);
+  border: none;
+}
+
+.ui.tertiary.button:hover {
+  color: var(--color-text);
+}
+
 .ui.primary.label,
 .ui.primary.labels .label {
   background-color: var(--color-primary) !important;