Fixes to dark mode (256 color scheme)

[ilya40umov]

Fixes for the following problems:

• terminal: highlight in ctrlp (dark scheme) not visible #64 (highlight not visible when using ctrlp/nerdtree)
• Wrong VertSplit after PR #58 #115 (vertical split not visible, e.g. border between nerdtree and main buffer not visible)
• The text color on the vim autocompllete modal are hard to see #109 (the text colors on autocomplete are hard to see)
• background of autocomplete / selected item for dark mode (256 color scheme) are the same as the overall background

[rvighne]

Due to inactivity from the maintainers, I've merged this (and some other long-standing PR's) into my fork: rvighne#4