Replace theme.js to theme.ts

[bertearazvan]

As our boilerplate is based on Typescript, we should make sure that all of the files that we bootstrap are based on Typescript.

One of the files that are currently not using Typescript is theme.js. While trying to migrate this file to using .ts we have encountered the following problems in regards to the types:

• Not using custom palette keys such as "medium". We should focus only on the given options.
• Using themeName throws errors as it is not part of the Theme type. I believe this is because themeName might be deprecated after some materialUI version (import { Theme } from ".../core"; - for the Theme types/interface.
• The overrides contains MaterialUI pickers classes which are not included in the general materialUI classes. I believe the best way to deal with this is either remove it and to keep only materialUI classes and eventually keep a log of the pickers classes. Or maybe add the pickers/types as peerDependencies?
• There are some errors with the '&hover' events.