Hey there, There is not a way to individually change the color of Show All History but you can go to customize and then themes. There you can change the theme.

Neither messing with the 'userChrome.css' file?

If you already are familiar with userChrome.css, there probably are some rules you could use for the background of that window. I assume if you make the background black you need to change the text color as well. That is where it starts to get tricky because the "tree" structures used for bookmarks and history are difficult to style.