This a child of the default 2015 theme. It:

• enqueues the parent theme file
• enqueues a main.css file which includes any custom CSS
• in functions.php it unplugs 2015's twentyfifteen_fonts_url function so it doesn't load any external fonts
• includes a main.scss file which should be compiled to main.css

• Select the Wp Base theme from Appearance > Themes
• Add your own CSS to main.scss (probably via SASS includes)
• Compile main.css from main.scss

WP Base generally makes no assumptions about your child theme. However, it comes with a few variables you can use throughout your CSS (if you're using SASS). These are:

• $main-font
• $secondary-font
• $mob-large
• $tab-small
• $tab-large
• $desk-small
• $desk-med
• $desk-large
• $desk-xl

See https://wordpress.org/support/topic/media-query-breakpoints for a list of Twenty Fifteen breakpoints.