[Web] Update Bootstrap to 5.3.8, add theme support

data/web/lang/lang.en-gb.json

@@ -405,6 +405,11 @@
         "ui_header_announcement_type_info": "Info",
         "ui_header_announcement_type_warning": "Important",
         "ui_texts": "UI labels and texts",
+        "ui_theme": "Theme",
+        "ui_theme_dark": "Includes a dark mode variant",
+        "ui_theme_default": "Default (mailcow)",
+        "ui_theme_info": "Themes are folders in <code>data/web/css/themes/&lt;name&gt;/</code> containing a <code>theme.css</code>. Optional: <code>dark.css</code> (dark mode), <code>bootstrap.css</code> (replaces the Bootstrap base theme) and <code>theme.json</code> (<code>name</code>, <code>description</code>).",
+        "ui_theme_none": "No themes found.",
         "unban_pending": "unban pending",
         "unchanged_if_empty": "If unchanged leave blank",
         "upload": "Upload",
@@ -554,6 +559,7 @@
         "webauthn_authenticator_failed": "The selected authenticator was not found",
         "webauthn_publickey_failed": "No public key was stored for the selected authenticator",
         "webauthn_username_failed": "The selected authenticator belongs to another account",
+        "ui_theme_invalid": "Theme not found",
         "unknown": "An unknown error occurred",
         "unknown_tfa_method": "Unknown TFA method",
         "unlimited_quota_acl": "Unlimited quota prohibited by ACL",
@@ -1214,6 +1220,7 @@
         "tls_policy_map_entry_deleted": "TLS policy map ID %s has been deleted",
         "tls_policy_map_entry_saved": "TLS policy map entry \"%s\" has been saved",
         "ui_texts": "Saved changes to UI texts",
+        "ui_theme_modified": "Theme was saved successfully",
         "upload_success": "File uploaded successfully",
         "verified_fido2_login": "Verified FIDO2 login",
         "verified_totp_login": "Verified TOTP login",