Hi there - I'm glad you were able to purchase the domain you wanted!
In terms of the theme, it depends what kind of changes you want to make. You would definitely be able to make certain design tweaks through the Custom Design upgrade, which is included in your bundle. For example, using CSS code, you could hide certain elements, change the font of your headings and body text, or modify background colours or images.
Premium themes are refundable for up to 30 days, so if you purchased the theme and it turned out you weren't able to make the kinds of changes you wanted, you would be eligible for a refund within that period. Or if you let me know the kind of changes you had in mind first, I'd let you know whether they're possible through the Custom Design upgrade.