Redesign country dropdown to fit redesign

This tweaks colors of the country dropdown to blend in better with the rest of
the design.

Fixes https://github.com/vector-im/riot-web/issues/9048

res/css/views/auth/_AuthBody.scss

@@ -80,6 +80,10 @@ limitations under the License.
         background-color: $authpage-primary-color;
     }
 
+    .mx_Dropdown {
+        color: $authpage-primary-color;
+    }
+
     .mx_Dropdown_arrow {
         background: $authpage-primary-color;
     }