Dear @mirjam
Is it possible that you have set two colours? one within your frontpage's gridder 'set background colour' and the other within Customize > background > background color.
I see on your website that your Grid is set to #5555f1
And your website background to ##ffffff
Or are you having difficulty just changing one of them - this is not working?
Site Title text is currently set to 18px - what were you trying to have?
Best wishes :)
Richard