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If you do all those changes you should be on track but it’s hard to say really until do it on the theme if that makes sense? Just with the CSS changes remember CSS is cascading and some themes use !important to over-ride this. It’s best to just in testing add your new CSS to the last CSS file called to make sure it has a fighting chance (key term in CSS is cascading). Also consider the theme may have it’s own styling different from the default theme even if uses the basic structure of it.