Changing the default font

I’m running RichWP MasterGrid theme and wondered how you change the default editing font?

When I publish a post, such as this one:

the font I want actually appears on the site, but the editing font when I’m writing a post is too small. I have recently moved my site to being self-hosted but I’m having to use my former blog (Twenty Thirteen theme) through WP to edit posts as the editing font there was right.

I have fiddled with the ‘Editor’ code under ‘Appearance’ but nothing has changed. I think that the font I’m after is Source Sans Pro, but I can’t be sure (see link above).

Many thanks,

Andy.