Centred heading and menu?

Hi, I am new to WordPress, have only been using it for a few days. I am also new to CSS. I would like to have my heading/site title and tagline text centred on the page and stretched so the tagline is only on one line not two lines, and also my top menu centred and stretched so it’s only on one line not two lines. Is this possible? I have looked at other replies and tried adding this one in the Customise/Additional CSS section, but I got an error message saying that it’s missing a colon. Any help (in detail because I’ve never done this before!) would be greatly appreciated. Thankyou!

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