[FEATURE] Different shading factor for bright/dark theme

[gmmyung]

My Neovim setup changes its theme based on its OS dark mode setting. The main problem is that toggleterm hardcodes the shading factor of light/dark background.

the percentage by which to lighten terminal background, default: -30 (gets multiplied by -3 if background is light)

This results in bright mode background clipping to plain white, which isn't desired. It would be nice to separate those two shading factor, or changing the shading algorithm so that it doesn't clip into plain white.