feat(themes): theme top-level record

HellionChat/Themes/Theme.cs

@@ -0,0 +1,12 @@
+namespace HellionChat.Themes;
+
+public sealed record Theme(
+    string Slug,
+    string Name,
+    string Author,
+    string Description,
+    ThemeColors Colors,
+    ThemeLayout Layout,
+    ThemeTypography Typography,
+    bool IsBuiltIn
+);