Improve migrate page and add card CSS (#13751)

- Use original gitea logo on migrate page
- Add card styles and map colors to css vars
- Tweak migrate page, adding hover effect to cards

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/_base.less

@@ -94,10 +94,12 @@
   --color-hover: #0000000a;
   --color-active: #00000010;
   --color-menu: #ffffff;
+  --color-card: #ffffff;
   --color-markdown-table-row: #00000008;
   --color-markdown-code-block: #00000010;
   --color-button: #ffffff;
   --color-code-bg: #ffffff;
+  --color-shadow: #00000024;
 }
 
 :root:lang(ja) {
@@ -374,6 +376,48 @@ a.muted:hover,
   box-shadow: -1px -1px 0 0 var(--color-secondary);
 }
 
+.ui.cards > .card,
+.ui.card {
+  background: var(--color-card);
+  border: 1px solid var(--color-secondary);
+  box-shadow: none;
+}
+
+.ui.cards > .card > .content,
+.ui.card > .content {
+  border-color: var(--color-secondary);
+}
+
+.ui.cards > .card > .extra,
+.ui.card > .extra,
+.ui.cards > .card > .extra a:not(.ui),
+.ui.card > .extra a:not(.ui) {
+  color: var(--color-text);
+}
+
+.ui.cards > .card > .extra a:not(.ui):hover,
+.ui.card > .extra a:not(.ui):hover {
+  color: var(--color-primary);
+}
+
+.ui.cards > .card > .content > .header,
+.ui.card > .content > .header {
+  color: var(--color-text);
+}
+
+.ui.cards > .card > .content > .description,
+.ui.card > .content > .description {
+  color: var(--color-text);
+}
+
+.ui.cards a.card:hover,
+.ui.link.cards .card:not(.icon):hover,
+a.ui.card:hover,
+.ui.link.card:hover {
+  border: 1px solid var(--color-secondary);
+  background: var(--color-card);
+}
+
 .ui.progress[data-percent="0"] .bar .progress {
   color: var(--color-text);
 }