The font used in the screenshot is Fira Mono.
Customization of the One Dark Syntax theme, adjusted to work well with SuperCollider by highlighting symbols and argument tags.
One Dark Syntax has a matching UI theme.
This theme is installed by default with Atom and can be activated by going to the Settings > Themes section and selecting it from the Syntax Themes drop-down menu.
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.