phloo I'm not sure I agree. It's very common for disabled buttons to be just a bit greyed out, like bootstrap does. But because in Flarum we can customize the primary color, I also understand there might be color choices where it isn't as good as others.
To change it on your own forum, you could add custom CSS for the admin area via a call in
extend.php. An easy option is do something like this:
// Add if not already there:
// Keep existing calls that might be here
->css(__DIR__ . '/admin.less'),
// your custom style
There should also be a way to insert inline CSS right in
extend.php but I can't recall the syntax right now. Maybe someone else can post it here for reference.