Improve CSV rendering (#29638) (#29744)

Backport #29638 by @silverwind

Before:

<img width="1332" alt="Screenshot 2024-03-06 at 21 42 17"
src="https://github.com/go-gitea/gitea/assets/115237/0ea07eee-31f8-4783-bd56-37bd8396f00d">

After:
<img width="1336" alt="Screenshot 2024-03-06 at 21 41 58"
src="https://github.com/go-gitea/gitea/assets/115237/eb7f9cc9-587f-4e3b-92bd-cc67ca639963">

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/repo.css

@@ -1432,6 +1432,7 @@
 
 .repository .data-table tr {
   border-top: 0;
+  background: none !important;
 }
 
 .repository .data-table td,
@@ -1444,6 +1445,21 @@
   border: 1px solid var(--color-secondary);
 }
 
+/* the border css competes with .markup where all tables have outer border which would add a double
+   border here, remove only the outer borders from this table */
+.repository .data-table tr:first-child :is(td,th) {
+  border-top: none !important;
+}
+.repository .data-table tr:last-child :is(td,th) {
+  border-bottom: none !important;
+}
+.repository .data-table tr :is(td,th):first-child {
+  border-left: none !important;
+}
+.repository .data-table tr :is(td,th):last-child {
+  border-right: none !important;
+}
+
 .repository .data-table td {
   white-space: pre-line;
 }
@@ -1481,7 +1497,7 @@
   min-width: 50px;
   font-family: monospace;
   line-height: 20px;
-  color: var(--color-secondary-dark-2);
+  color: var(--color-text-light-1);
   white-space: nowrap;
   vertical-align: top;
   cursor: pointer;