doc: make legacy banner slightly less bright

PR-URL: #44665
Reviewed-By: Beth Griggs <bethanyngriggs@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: Trott

doc/api_assets/style.css

@@ -4,7 +4,7 @@
   --black1: #090c15;
   --black2: #2c3437;
   --black3: #0d111d;
-  --blue1: #00f;
+  --blue1: #0a56b2;
   --white: #ffffff;
   --white-smoke: #f2f2f2;
   --grey-smoke: #e9edf0;