Add an ability to change theme of the app

[pavelkeyzik]

In src/index.tsx we have code like this

<ThemeProvider colorMode="auto">
  {...}
</ThemeProvider>

But we don't have any place to change it, as for example in my case, auto doesn't work and I see white theme instead of dark. It some issue with Chrome on Ubuntu, but I think it's still nice to have that button as sometimes user can prefer white theme even though the system preferences are set to Dark.