Fix some button border colors in dark mode

Remove border-white-light - it was only used by
emoji-menu which is now disabled by default.

The previous color was much too light, we can just
use the default border color instead. I'd ideally
like to remove the $border-white-normal variable
from light mode as well but maybe later.

Changelog: fixed

app/assets/stylesheets/startup/startup-dark.scss

@@ -480,7 +480,7 @@ body {
 .btn:active,
 .btn.active {
   background-color: #444;
-  border-color: #fafafa;
+  border-color: #4f4f4f;
   color: #fafafa;
 }
 .btn svg {

app/assets/stylesheets/themes/_dark.scss

@@ -186,8 +186,7 @@ body.gl-dark {
   }
 }
 
-$border-white-light: $gray-900;
-$border-white-normal: $gray-900;
+$border-white-normal: $border-color;
 
 $body-bg: $gray-10;
 $input-bg: $white;

ee/app/assets/stylesheets/startup/startup-dark.scss

@@ -480,7 +480,7 @@ body {
 .btn:active,
 .btn.active {
   background-color: #444;
-  border-color: #fafafa;
+  border-color: #4f4f4f;
   color: #fafafa;
 }
 .btn svg {