This is Simplified Color, an alternative theme to Simplified Dark and Light in Textual. It provides the same simplistic user interface seen in Simplfiied with a few colorful enhancements.

The theme works only on versions of Textual 4.1.4 or later.

If you would like to install this theme, download the zip and unzip it. You then install it following this filepath: ~/Library/Containers/com.codeux.irc.textual/Data/Library/Application Support/Textual IRC/Styles/. If you don't have the Library folder showing by default, click "Go" on the menu bar in Finder and press the option key to get to the Library folder.

Here's a look at both the dark and light themes - you get both at once upon downloading: