add theme override customizations

LightlessSync/LightlessConfiguration/Configurations/UiThemeConfig.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+
+namespace LightlessSync.LightlessConfiguration.Configurations;
+
+[Serializable]
+public class UiThemeConfig : ILightlessConfiguration
+{
+    public Dictionary<string, UiStyleOverride> StyleOverrides { get; set; } = new(StringComparer.OrdinalIgnoreCase);
+
+    public int Version { get; set; } = 1;
+}