Deeply merging enhancement

> Deeply merging: theme objects are merged and if a key is present in more than one object, the values for each objects are concatenated.

While this does offer the most flexibility, I think it can be improved to override styles set by parent themes/defaults.

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Take this example. I think it would be cool to see specific nested styles defined in **global** overwritten if also defined by **specialCase**.

@javivelasco If I am understanding correctly, this might be pending work on **postcss-apply**?

Post Edit:

Note current work arounds are to use selectors that are more specific and/or override using `!important`.

Start by tracing the existing theme merge behavior around the nested ThemeProvider example and review the referenced postcss-apply work. Done means nested styles defined by specialCase override the corresponding global or default styles while unrelated theme values continue to merge.

Written by the indexing model from the issue text.