Typogram images fail badly in dark mode

[tabatkins]

Currently, typogram's styles explicitly use black as the fill/stroke for all of its diagram bits. Since it also uses the default transparent bg for its generated <svg> element, this means that a page in dark mode sees black lines on the dark-mode background (generally black as well). You can see this in action at https://fedidcg.github.io/FedCM/#introduction (scroll down a little in the Introduction section). (I just submitted a PR overriding their styles to be darkmode-friendly, so depending on when you read this it's possible they've been fixed already. But you can just generate any diagram yourself and view it on a darkmode page.)

The easiest fix for this is to just change all those instance of black to currentcolor. This'll automatically adapt to the color scheme, and give users of the library a little bit of extra control if they wish.