Reuse CSS variables provided by django

[VaZark]

My browser defaults to prefer dark but when I toggle to light mode on Django 5.0, I notice that certain elements remain on dark mode colors.

Taking a look at the CSS, I realise that the colours are hard-coded. Django has migrated to using CSS variables for the admin page sometime during the 3.0 release cycle. Reusing the same variables in the libraries CSS would make the entire experience more consistent with upstream OOTB. It also reduces overhead as those who wish to merely customise the colours can modify the default variables and forget the rest.

[VaZark]

I can take a look and send in a PR sometime over the month if you're open to it