feat: Add theme prop (#3228)

* support appearance when updating scene data

* works!

* whoops, missed a prop

* hide appearance button when prop is not set

* cleanup

* fix export + rename prop to theme

* rename to showThemeBtn, hide via react instead of css

* adapt to new state name

* add tests and css selector to target the dark mode toggle

* updated changelog and readme

* fix markdown rendering in readme

* pr feedback

src/types.ts

@@ -190,6 +190,7 @@ export interface ExcalidrawProps {
   zenModeEnabled?: boolean;
   gridModeEnabled?: boolean;
   libraryReturnUrl?: string;
+  theme?: "dark" | "light";
 }
 
 export type SceneData = {