Adding a create theme utility function

[ankibalyan]

Currently, When it is required to create a new theme we need to explicitly create a new theme and copy paste add all the necessary things from the base theme.
Or we need to clone a base/ light/ dark theme and then merge to them.

what about create a utility function similar to material-ui to extends one of its core theme safely along with all the components CSS so things doesn't break.

[ankibalyan]

Or may be adding an example of extending to a full new theme to docs may also help.

[siddharthkp]

I'm slightly skeptical about encouraging folks to extend the in built theme

1. on one hand, if you only want to change one thing, it would be annoying to copy paste the whole thing
2. on the other, if you want to refer to tokens, which means you'll have to dig into the theme to find the correct keys, making the theme yours by copy pasting makes that path easier

That said, definitely needs a page in the docs when we have a solid opinion