Styles Twenty Thirteen not installing (even though I've installed it!)

Hi, I’m completely new to all this and would appreciate some help.

I have installed Styles: Twenty Thirteen to change the headings etc on my Twenty Thirteen theme – it will change everything then suddenly it reverts back to the original settings and tells me that ‘Styles: Twenty Thirteen is nearly ready to install’ even though it’s already there.

This keeps happening – one minute all my changes have gone, then they come back … I also just have ‘styles’ installed – maybe it conflicts?!