Handle the case when a theme file already exists

[umaar]

When a theme file already exists, you see this message:

A theme file already exists in ....src/themes/theme.ts. Will not overwrite.

But we do still generate the CSS modules. We should address this somehow, for example, print out the code in the terminal, and instruct the user to merge it with their existing theme file.

[agubler]

I think we should just exit if the theme already exists....