You are using the mobile theme for mobile devices, so you don't need to worry about the mobile setup — your blog's custom CSS won't be included in the theme that shows up on mobile devices unless you choose to have it used there.
In the Confit theme, you can control the maximum width of the content area by changing this CSS:
The code above is the current setting. You can change the number of pixels in that code to make the maximum width as big as you'd like. If you want it to be as wide as possible in any browser size, you can change it to 100% like this:
You may also want to take a look at our information about CSS at these links:
If you have any other questions about customizing your blog, most of our CSS support is done in our community forums by both staff and volunteers:
I hope this helps!