fix(web): fix Theme Custom CSS endpoint requiring the user to be logged in as the server admin (#4633)

* fix custom css requiring the user to be the admin and logged in

* move theme api to custom endpoint

* add e2e test

mobile/openapi/lib/model/server_theme_dto.dart

@@ -0,0 +1,98 @@
+//
+// AUTO-GENERATED FILE, DO NOT MODIFY!
+//
+// @dart=2.12
+
+// ignore_for_file: unused_element, unused_import
+// ignore_for_file: always_put_required_named_parameters_first
+// ignore_for_file: constant_identifier_names
+// ignore_for_file: lines_longer_than_80_chars
+
+part of openapi.api;
+
+class ServerThemeDto {
+  /// Returns a new [ServerThemeDto] instance.
+  ServerThemeDto({
+    required this.theme,
+  });
+
+  SystemConfigThemeDto theme;
+
+  @override
+  bool operator ==(Object other) => identical(this, other) || other is ServerThemeDto &&
+     other.theme == theme;
+
+  @override
+  int get hashCode =>
+    // ignore: unnecessary_parenthesis
+    (theme.hashCode);
+
+  @override
+  String toString() => 'ServerThemeDto[theme=$theme]';
+
+  Map<String, dynamic> toJson() {
+    final json = <String, dynamic>{};
+      json[r'theme'] = this.theme;
+    return json;
+  }
+
+  /// Returns a new [ServerThemeDto] instance and imports its values from
+  /// [value] if it's a [Map], null otherwise.
+  // ignore: prefer_constructors_over_static_methods
+  static ServerThemeDto? fromJson(dynamic value) {
+    if (value is Map) {
+      final json = value.cast<String, dynamic>();
+
+      return ServerThemeDto(
+        theme: SystemConfigThemeDto.fromJson(json[r'theme'])!,
+      );
+    }
+    return null;
+  }
+
+  static List<ServerThemeDto> listFromJson(dynamic json, {bool growable = false,}) {
+    final result = <ServerThemeDto>[];
+    if (json is List && json.isNotEmpty) {
+      for (final row in json) {
+        final value = ServerThemeDto.fromJson(row);
+        if (value != null) {
+          result.add(value);
+        }
+      }
+    }
+    return result.toList(growable: growable);
+  }
+
+  static Map<String, ServerThemeDto> mapFromJson(dynamic json) {
+    final map = <String, ServerThemeDto>{};
+    if (json is Map && json.isNotEmpty) {
+      json = json.cast<String, dynamic>(); // ignore: parameter_assignments
+      for (final entry in json.entries) {
+        final value = ServerThemeDto.fromJson(entry.value);
+        if (value != null) {
+          map[entry.key] = value;
+        }
+      }
+    }
+    return map;
+  }
+
+  // maps a json object with a list of ServerThemeDto-objects as value to a dart map
+  static Map<String, List<ServerThemeDto>> mapListFromJson(dynamic json, {bool growable = false,}) {
+    final map = <String, List<ServerThemeDto>>{};
+    if (json is Map && json.isNotEmpty) {
+      // ignore: parameter_assignments
+      json = json.cast<String, dynamic>();
+      for (final entry in json.entries) {
+        map[entry.key] = ServerThemeDto.listFromJson(entry.value, growable: growable,);
+      }
+    }
+    return map;
+  }
+
+  /// The list of required keys that must be present in a JSON.
+  static const requiredKeys = <String>{
+    'theme',
+  };
+}
+