MarketPress implementation as a 1995 online boutique

I got a small ecommerce contract for that client. So I add a /boutique on the website.

I installed and configured MarketPress and guess what, my boutique looks like my websites of 1995.

I think I have more than one issues that may or not be related together...

If you reach:
You can observe what I mean. Like no style sheet at all.

Also, if you go down the road to
I get a white screen. Else then the error, I would also need help to find how to remove the "Category" level from the url.

Thanks for your help!