Yes, it is. There haven't been major changes in HTML/CSS theme structure in 1.8, so most things should work fine. Note if you have installed a plugin that adds frontend code, you might find some glitches, but that applies to all themes, of course.
You can test it yourself downloading it from here: https://github.com/amiyasahu/Donut/archive/master.zip . If anything goes wrong, you should be able to go back to your previous theme.
