Temporary theme from current theme

Howdy, I am working on a 4th July theme for my forum. However, I would like to keep my current somewhere when I change over the new look.  DO I export my current form and save it that way?

I also don't want to change it over if its going to mess up the Categories and Forum threads. Is that always going to stay the same when you change over Themes?

When I am working on a temporary theme, to get ready to be the current, do I just apply this theme to the fourm. The banner isn't centered would that be fixed when I apply this theme to the forum?

Is the portal page going to be the same too or do I have to make new one when I use a new theme.?

I would like to get my new theme up before July 1st.
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Thank You,

Jen