[FreddleSpl0it]

Contribution Guidelines

• I've read the contribution guidelines and wholeheartedly agree them

What does this PR include?

Short Description

Modernizes the UI stack and adds a theme system, without changing the default look.

Bootstrap 5.1.3 → 5.3.8

• Bootstrap bundle 5.3.8 and Bootswatch Lumen 5.3.8
• bootstrap-select CSS 1.14.0-beta
• 5.3 tab fixes

Theme support

• Colors in 014-mailcow.css, 013-datatables.css and mailcow-darkmode.css are exposed as --mc-* custom properties, default values unchanged
• Theme folders in data/web/css/themes/<name>/: theme.css (required, loaded after all mailcow styles), optional dark.css, bootstrap.css and theme.json (name, description)
• Themes can be selected in System > Configuration > Customize
• The legacy way ($UI_THEME + css/themes/<name>-bootstrap.css) keeps working unchanged
• Dark mode is now bundled into /cache/<hash>-<darkhash>.css like the light CSS.

servercow theme (css/themes/servercow/)

• Based on the servercow theme

Login pages

• Colored marker per role on the login card (user: green, domain admin: yellow, admin: red), colors overridable via --mc-login-*
• Role specific header icons (person / globe / shield), so the roles stay distinguishable for color blind users

Affected Containers

• nginx-mailcow
• php-fpm-mailcow

Did you run tests?

What did you tested?

• Upgrade path for existing users: Bootswatch Flatly 5.1.3 via $UI_THEME in vars.local.inc.php plus custom rules in 0081-custom-mailcow.css
• Theme selection: select theme, switch back to default, invalid theme name via API
• Login pages for all three roles, light and dark, incl. color blindness simulation