Google Chrome lets you add a cast button to the toolbar, nice when you have a Chromecast. Those who want one in Vivaldi are left lacking.... or are they?...

How to add a Chromecast button in Vivaldi
Go to settings, quick commands, scroll down to command chains.
In there click the plus icon, give the thing a name, Cast, and select the command Cast.
DO NOT click test chain, that'll crash the browser. Made me think it didn't work.. it does.
Just close the settings window.
Then right-click on your profile image, customize toolbar.
In the drop-down click Command Chains, you'll find the one just created, and can drag it to the spot you want, and click done.
Could be as easy as that, and you're done.
But I wanted a cast icon, so I got a 28x28 fitting bitmap thingie from here: https://developers.google.com/cast/docs ... s_optional
I edited the color I wanted (grey) in Gimp since I wanted a connected icon, but with an open inside, just like in Chrome. Maybe I didn't look far enough into the folders, could have been there already, anyway, I made one.

Then go to settings, click themes.
Click the Editor tab, and then about 4 lines down or so there is another line of tabs, one says icons, click that.
Scroll all the way down, and there is your creation. Click that, and click on the 28x28 image to put a new one in there, done.
Close the settings window.
Now right-click on your profile image, customize toolbar etc., drag it in place, click done, and voilà: one cast icon in my toolbar now:

When I click on it, I get the same pop-up window I get when I go into the menu on the top left, File - Cast..