The primary stylesheet you want to modify is here:
The first thing I would do is simply remove \wp-content\themes\style.css from your server, by downloading it locally (e.g. via FTP). Then, see what impact it has on your site.
If you see drastic changes, then you want to copy the content of the removed style.css file into
\wp-content\themes\learningmatters\style.css, preferably before any other style declarations but (very important) after the header block and any calls to @import.
The header block is this bit:
Theme Name: Learning Matters
Description: This is the theme for Learning Matters.
It appears that you don't have any @import calls or any actual style definitions in this file.
So, it is safe to do the following:
\wp-content\themes\style.css in a text editor
2) Copy the entire contents of this file
\wp-content\themes\learningmatters\style.css in a text editor
4) Paste the contents of the previous file into this file, after the header block.