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/components/Modal.tsx

@@ -6,6 +6,7 @@ import clsx from "clsx";
 import { KEYS } from "../keys";
 import { useExcalidrawContainer, useIsMobile } from "./App";
 import { AppState } from "../types";
+import { THEME } from "../constants";
 
 export const Modal = (props: {
   className?: string;
@@ -15,7 +16,7 @@ export const Modal = (props: {
   labelledBy: string;
   theme?: AppState["theme"];
 }) => {
-  const { theme = "light" } = props;
+  const { theme = THEME.LIGHT } = props;
   const modalRoot = useBodyRoot(theme);
 
   if (!modalRoot) {