Fixes sharkdp#328 - adds info for light backgrounds.

README.md

@@ -210,6 +210,14 @@ the following command (you need [`fzf`](https://github.com/junegunn/fzf) for thi
 bat --list-themes | fzf --preview="bat --theme={} --color=always /path/to/file"
 ```
 
+`bat` looks good on a dark background by default. However, if your terminal uses a
+light background, some themes will work better for you. For example, the `GitHub`
+theme is made for light backgrounds. To set your theme permanently, put
+`export BAT_THEME="GitHub"` in your shell's startup file (like `.bashrc`). If you'd
+like to see the other available themes, run `bat --list-themes`. You can also make
+your own theme by following the
+['Adding new themes' section below](https://github.com/sharkdp/bat#adding-new-themes).
+
 ### Output style
 
 You can use the `--style` option to control the appearance of `bat`s output.