The width of the main column is defined here:
margin: 0 auto;
You can modify the max-width to something else if you like.
There is also another relevant width defined here that you can adjust as well:
Don't edit the theme files directly, otherwise your changes will be overwritten when the theme is updated.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You'll then add your custom CSS in the new stylesheet editor that'll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.