🗑️(frontend) define SASS color variable out of bootstrap v5

$table-head-color used in saas fil is not set-up anymore. The upgrade of the
library bootrstrap version 4 to 5 doesn't have this variable pre-configured
anymore. As a result, the frontend won't be able to be build anymore. We fix
this issue by giving a color to this variable so that Ashley can use bootstrap
5.

src/frontend/scss/_variables.scss

@@ -9,5 +9,6 @@ $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%);