Post page font is too small, and post layout

I’m using the free Leto version, and everything looks good, except for the blog post.

  1. Font size is too small for the blog. When I try to customize the CSS for the post page, I found out that some words on the checkout page were changed too, which means there are two different font sizes on the checkout page, which looks not nice.

So the question is how I can only customize the font family and font size for the BLOG POST, but not affect PAGE by adding additional CSS?

  1. How to make the post page looks more professional?
    How to improve it by adding additional CSS?
    I want something like this:

At least two things,

  • How to customize the post page sidebar? The sidebar on post page is same as the category page now.

  • Width of the post content is a bit big, which is not good for reading, how I can make it 700px?