Headings Font Size option (for slider)

Hello, (new.gpf1.cz) within 'Font Size Options' I can find 'Headings Line Height' only (not Size, despite included in Theme Instructions).There are no Font Size options within 'Featured Slider' option either. My goal is (1) to change Headings Font Size (2) to give it some background so the Text is distinguishable from the image itself (e.g. like on www.gpf1.cz main page) - because, no matter what color I chose, Headings text is not sharp enough to be easily readable and eye-catching. Thanks for advise Tom