nk_style_push_X is defined, but never actually implemented

EDIT: https://github.com/Immediate-Mode-UI/Nuklear/issues/310#issuecomment-869039730

🤔
```
$ grep nk_label_colored_wrap nuklear.h
NK_API void nk_label_colored_wrap(struct nk_context*, struct nk_slice, struct nk_color);
```

Start in nuklear.h by checking the nk_style_push_X declaration and the nk_label_colored_wrap declaration shown in the issue. Reconcile the title with the body, identify which declared API is missing its implementation, and verify that the completed change provides the expected implementation without leaving the declaration unresolved.

Written by the indexing model from the issue text.