Clone sub directories into ~/Library/Containers/com.codeux.irc.textual/Data/Library/Application Support/Textual IRC/Styles
For instance, the Xarno theme would live at ~/Library/Containers/com.codeux.irc.textual/Data/Library/Application Support/Textual IRC/Styles/Xarno
Then choose the Style from the Textual Preferences menu.
Upgraded theme for support of Textual 5. Previous versions can be found on the long lived branch textual4
.