By default, in the Extra Theme, the titles on archive pages (category pages, tag pages, etc.) are left-aligned. To center-align these archive page titles instead, you can use this CSS code:

/* Center align the title on archive pages */
body.archive .et_pb_extra_column_main > h1 { 
	text-align: center; 
}

To add the CSS code to Extra, paste it into Extra's custom CSS box, which you can find from your WordPress admin menu at "Extra > Theme Settings > General > Custom CSS". Paste it after any other code already in there.