Site title and slogan, centered

Hello,
I would like to have the title and slogan of my website showing in the center of the background photo.
When I go to Appearance/Customize/Header/Main header , I choose the position “center” and nothing happens.
Am I missing something? or should I create additional CSS to get there?
If I need CSS, could you please help me figure out how to get the right css…. I have tried but couldn’t get it centered….
Thanks in advance for your help

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