archive issue need help ;/

I have been working on this site. I created a custom there via a tutorial and everything seems okay except that the archives’ css seems completely wrong once you access the sites archived blog posts. Where would the template be or is this 100% pure CSS?

so if you go into the blog post single it is fine ..if you then go into that archive (from the widget on the sidebar) everything goes wacky any suggestions ??

https://www.biancabarnett.com/2011/02/06/