feat: add splash window theming (#52)

Co-authored-by: V <vendicated@riseup.net>

src/shared/settings.d.ts

@@ -26,4 +26,8 @@ export interface Settings {
 
     skippedUpdate?: string;
     firstLaunch?: boolean;
+
+    splashTheming?: boolean;
+    splashColor?: string;
+    splashBackground?: string;
 }