I just checked in the changes that I did for my forum to make YAF.NET responsive here:https://github.com/Redyta/YAFNET
Take a look and let me know if there are any issues with running it.
I only applied theme changes to BlackGrey theme.
To make responsive work with images, I made some changes so they stretch to width. There is an issue with small images that get starched so they don't look good but I didn't want to spend more time on that.
Also, signature gets displayed right after message instead of bottom of the row. Again, I didn't want to spend time on that.
Other than that, the main pages of BlackGrey theme should look OK on all devices.