I'm so excited for you! I know you will have a wonderful time. I took my first Rick Steves Tour as an experiment, "is group travel a match for me?" I loved that tour. I signed up for another tour the next year and enjoyed it even more. I had one cancelled last year but am signed up for Sept 2022.
I really don't think anyone paid a great deal of attention to anything resembling a "dress code." People dressed in clothes that were comfortable for them and could be versitile in multiple settings. Some of the women looked "put together" no matter where we were or what we were doing. I have always admired those women! The rest of us dressed for comfort and utility. No one seemed to care either way.
If you are a solo traveler, it is hard to tell from your post, I do have a little advice for you.
Get the single supplement, be willing to reach out to others, and have plans you can do independently during free time determined in advance. Chances are good you will find others wanting to do something together, but if not, be prepared to enjoy yourself solo.
I have found the people on Rick Steves tours to be exceptionaly agreeable and enjoyable to travel with. I always learn a great deal from the guides and feel like I get a good value from the tour.