Vivaldi browser is still in technical preview, its company started to add features to it, are you missing extensions? Don’t worry! You can install apps, extensions and themes from Chrome Web Store in Vivaldi browser and they work like in Google browser.
How to install extensions in Vivaldi?
Visit Chrome Web Store page in Vivaldi, search for an extension you’re using in the Chrome browser and install them.
You can find the installed apps in Vivaldi://apps page and extensions in Vivaldi://extensions page, and you can uninstall them by visiting those pages.
At present, there is no way to revert the installed theme to default one, so you should know about that before installing a theme in this browser. This is the page, that lists the all internal URLs of Vivaldi.