Rewrite colour setting in _smart_apply

Summary:
Rewrite colour setting to be more general. Before colour inversion
did not work with 256 colour palette.

Test Plan: echo -e "\e[7;38;5;240;48;5;255m####\e[0m" --> output is invisible

Reviewers: billiob

Reviewed By: billiob

Differential Revision: https://phab.enlightenment.org/D421
remove unused variable