Currently, when playing the game in fullscreen-mode, the game won't actually run at your monitor's native resolution. There's a more in-depth explanation here, but the TL;DR is that, due to a bug, the game will:
- Render at the selected resolution (this is why screenshots come out looking fine)
- Downscale that to the nearest supported resolution on your monitor that isn't in your monitor's native aspect ratio (you can see what I mean by looking in the Windows advanced display settings dialogue on a second monitor if you have one), and then
- Your GPU or monitor upscales that again to the selected res.
To get the fix, download this file and extract its contents into your game's install directory: Link
When you boot up the game for the first time after this, there'll be some diagnostics text overlaid on top of it; hit CTRL + SHIFT + O to get rid of it and it'll stay gone even after you restart.
EDIT: If you're on a Windows version earlier than 8.1, you'll need two extra DLLs which you can get here: Link – grab the two files from the "x64" folder in this archive, and throw them in the game's install directory, same as the mod itself. If you're running Windows 8.1 or Windows 10, this is not needed.