feat: Export THEME from the package (#4055)

* Use Theme type everywhere
* Rename Appearance type to Theme for consistency
* Reorder headers in readme
The host don't need to pass hardcoded strings any more and instead can use the exported constant

src/appState.ts

@@ -4,6 +4,7 @@ import {
   DEFAULT_FONT_SIZE,
   DEFAULT_TEXT_ALIGN,
   EXPORT_SCALES,
+  THEME,
 } from "./constants";
 import { t } from "./i18n";
 import { AppState, NormalizedZoomValue } from "./types";
@@ -18,7 +19,7 @@ export const getDefaultAppState = (): Omit<
   "offsetTop" | "offsetLeft" | "width" | "height"
 > => {
   return {
-    theme: "light",
+    theme: THEME.LIGHT,
     collaborators: new Map(),
     currentChartType: "bar",
     currentItemBackgroundColor: "transparent",