Reintroduce themes to default_admin templates

[DaFa66]

On commit 52e9fbf, admin.css was thought to be a duplicate - it wasn't.

Changes made to all six admin themes:

• Renamed admin.css to admin-theme.css to avoid confusion
• Reintroduce <link rel="stylesheet" href="<?= THEME_DIR ?>css/admin-theme.css">

PLEASE NOTE Upgrading from the desktop App will not apply this change, unless you are creating a new app.

[trongate]

Good call! Thank you and Merry Christmas!