Fix selection join

Number of characters isn't same as number of bytes.
color/light256: Use terminal's default fg and bg colors

tmux doesn't support back_color_erase terminal capability. This means
that erase to end of line with bg color set needs to be emulated by
printing spaces. This breaks copying such lines using mouse because all
the spaces at end of line is copied too. Also printing lots of spaces
make screen update slower.

For this reason assume that terminal's default fg and bg colors are
suitable for this color theme.