Expose theme provider (#3001)

### Description
While working on the tests for a new internal component, I noticed that
it was very tricky to test without the Theme, so instead of trying to
mock it and try to fix every property that throws an error, we should
export it.

frontend/packages/core/src/index.tsx

@@ -60,6 +60,7 @@ export { default as TimeAgo } from "./timeago";
 export { Typography } from "./typography";
 export { default as ClutchApp } from "./AppProvider";
 export { useTheme } from "./AppProvider/themes";
+export { ThemeProvider } from "./Theme";
 
 export { css as EMOTION_CSS, keyframes as EMOTION_KEYFRAMES } from "@emotion/react";