I checked your account and I see that there is no file called style10.css. Because that’s the file the page also references, that’s why the styling doesn’t work.
I do see there is a file called Style10.css. Please note that our servers, like almost all web hosting servers, run on Linux, which has case sensitive file names. So if you rename the file Style10.css to style10.css, it should fix the design of your aboutus.html page.