_content/css: color of buttons and links, in auto (dark) theme

Fixes golang/go#69215

Change-Id: I708e7af6790969444afc0527775643d4c42c90c3
Reviewed-on: https://go-review.googlesource.com/c/website/+/611155
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Hyang-Ah Hana Kim <hyangah@gmail.com>

_content/css/styles.css

@@ -138,6 +138,7 @@
     --color-text-subtle: var(--gray-7);
     --color-text-alert: #e67193;
     --color-code-comment: var(--green-light);
+    --color-bright-text-link: var(--turq-med);
   }
 }