Specify light and dark themes simultaneously, and auto-switch based on system appearance

[yam-liu]

I want to specify light and dark themes simultaneously, and auto-switch based on the system's appearance like the default behavior.

[bash]

Since delta supports bat's BAT_THEME environment variable it might be a good idea to wait for bat to implement this first. I have opened a PR for this a while back: sharkdp/bat#2896.

The mentioned PR for bat adds two new options / environment variables:

• --theme-dark / BAT_THEME_DARK
• --theme-light / BAT_THEME_LIGHT