The "Themes" tutorial on The Brain website says to right click on the background to change the theme. However, it doesn't seem like I am able to do this using my Macbook, with the Mohave OS. And I cannot find any other option to do this. 

Is there something I am completely missing? Thanks in advance. 

Right click the plex background and it should be the second to last option on the right-click menu. Alternately TheBrain  > Brain Theme.
macOS 10.14.6
TheBrain 10.0.54

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