Para components should use `theme.fontSize.base` or `theme.fontSize.text`

Current behavior

When stylegudist is configured to use a font size for base and/or text to be anything other than 16px, the paragraph content generated from markdown does not use the specified font size.

Question: Is a markdown paragraph considered to be either base or text? I am assuming it should be text at the very least.

To reproduce

1. Config:

   复制

   module.exports = {
     theme: {
       fontSize: {
         base: 24,
         text: 36,
       },
     },
   };

2. Some markdown file:

   复制

   # I am an H1
   
   I am just a paragraph! My font size should be really big!

https://github.com/styleguidist/example/pull/8

Expected behavior

The generated paragraph content should use the theme provided.

Or there should be some explanation of how the "theme" is defined: How is a paragraph not "text" or "base"?