This is a Textual style is based on the built-in Simplified Dark style using colours from Rainglow's Halflife by Dayle Rees.
Download or clone this project into the Textual Styles folder, then it should appear in Textual's Style preferences.
Textual will look at one of two locations to locate custom styles.
A copy of Textual downloaded through the Mac App Store will look at the following path:
~/Library/Group Containers/8482Q6EPL6.com.codeux.irc.textual/Library/Application Support/Textual/Styles/
Otherwise, Textual will look at the following path:
~/Library/Group Containers/com.codeux.apps.textual/Library/Application Support/Textual/Styles/
This path can be easily opened in Finder by bookmarking the following URL: textual://custom-styles-folder
I love the Fira Code font and recommend using that - but this style should work with any font you've chosen.