Quick protip for anyone wishing to drag&drop their own SVG icons into Design mode and be able to change the fill:
If you're like me, you export your SVG icons with
a) an empty rectangle the size of the artboard to ensure correct placement when dropped into Sketch etc, and
b) a color applied to the icon so you can also export PNGs from the same artboard.
But to change their color in Design mode, they can't have those two things. It's also generally handy to clean this up for production, and it's the main difference between the "design" and "production" versions of Google's Material icon SVGs. Enter this terminal command, in a folder full of SVG files (original files backed up with .bak extension):
sed -i .bak 's/<path d="[^"]*"[^>]*fill="none"[^>]*>//g ; s/fill="[^"]*"//g' *svg
And you're good to go :)