`--theme` required for `bat`’s colours to be used

[nisbet-hubbard]

Currently, just exporting LESSCOLORIZER='bat' doesn’t cause bat’s default theme to be used. Only LESSCOLORIZER='bat --theme=default' would produce the same colours as bat does out of box.

[wofr06]

This was done on purpose to have output with reasonable contrast for both dark and light background. The default theme looks good on dark background, but (at least for me) the default theme with light background does not produce readable output (some chars are printed white on white). The chosen ansi theme is a compromise.