Ensure styles are individually clearable

[g-elwell]

What problem does this address?

Once you add a style in themer, there is no way to fully remove it from your customisations. Within style book, you can 'reset' to completely remove the customisation.

What is your proposed solution?

Update the UI so that each style value has a method of clearing/resetting it. This should fully remove the value from the theme.json if it is a user customisation.

[chrishall0]

current functionality of clear customisation component http://bigbite.im/v/Lyrl4V
wondering if this is what we're expecting UI wise and regarding icons/wording etc.
@jonnywatersbb