ux: new custom theme configuration format

* supports customize the commit graph

src/Models/CustomColorSchema.cs

@@ -0,0 +1,10 @@
+using System.Collections.Generic;
+
+namespace SourceGit.Models
+{
+    public class CustomColorSchema
+    {
+        public Dictionary<string, string> Basic { get; set; } = new Dictionary<string, string>();
+        public List<string> Graph { get; set; } = new List<string>();
+    }
+}