Centering the header and changing the font

I’ve just switched to Atout theme for my website aladyofleisure.com and think it’s great but I can’t find out how to centre my header A Lady of Leisure nor change the size of it? At the moment it says Welcome to A Lady of Leisure but ideally I’d like it to say A Lady of Leisure in larger type, centred to the photo below, and with a tag line underneath (the tagline in the customise options doesn’t seem to appear on the page). I suspect I need to do something with CSS and text-align but I’ve no idea where to put it or what the header is called in CSS (it is H1 or is that just for blog posts?) I’m an enthusiastic newbie so any help (and simple explanations!) would be gratefully received, many thanks.