🩹(frontend) define SASS color variable

$table-head-color used in saas fil wasn't set-up. With renovate updates service,
the frontend couldn't be build anymore. We fix this issue by giving a color to
this variable.

Author: carofun

src/frontend/scss/_variables.scss

@@ -9,5 +9,7 @@ $red: #fe3925;
 // Relative path the the fontawesome webfonts (from the compiled CSS file)
 $fa-font-path: '../font';
 
-$unread-color: lighten($red, 20%);
 $read-color: lighten($blue, 15%);
+$table-head-color:#495057;
+$unread-color: lighten($red, 20%);
+