[color-] make status colors more readable

[midichef]

In develop, but not in v3.0.2, the display of the last command is hard to read. It's cyan on light blue:

That's because color_longname is currently 6, which is cyan. It's hard to pick a text color that is readable against the light blue status bar and also readable against the green guide, so this PR splits color_longname into two settings, color_longname_guide and color_longname_status.

A couple of other color changes:
For the ascii8 theme: the progress indicator (loading...) is hard to read, as color_working is currently green, on a cyan background. So I've changed it to bold white. Of the 8 foreground color choices, magenta and yellow were a bit too faint, red looked like an error, and the rest were too hard to read.

Also for ascii8, longname is currently invisible, cyan on cyan. The new default color for color_longname_status fixes this too.

And the lower right corner of the guide box has a stray bracket character. It currently reads: toggle sidebar |[┘, which this PR changes to toggle sidebar |─┘.

The reason for choosing white for color_longname_status, is that it's readable against the light blue status bar for the default theme, as well as against cyan for the ascii8 theme. If there are better colors, please feel free to go with those.

[saulpw]

Thanks for this, @midichef! I've been annoyed by the lack of readability myself, and these are definite improvements.