Sir, I believe you couldn't find the configurations menu, I am glad to help you, to find the configurations menu:

1- I want you to pin the extension if you can't see it in the top right of your browser.
2- Click on the extension icon, it should create a separate tab, this tab is the configurations (or actions) menu you want.
3- To see your configurations being set in real-time, open the configurations tab and in another tab Whatsapp Web, whenever you change a configuration remember to switch quickly to the Whatsapp Web tab to see the changes reflected.

Finally, when you find the configurations you can change the themes to something that suits your taste and combine it with some backgrounds to turn Whatsapp web into an artwork. This extension is already done the only time I have to change something is when Whatsapp decides to change their website, forcing me to spend time deciphering the encrypted new structure of their website and how it operates. If there is something I failed to address, please let me know.