Microsoft promised they’ll add support for themes in Chrome Web Store to the new Edge browser, finally themes support has been added to the new Edge browser albeit in Canary behind a flag.
While the new Microsoft Edge Chromium already allows installing extensions from Chrome Web Store, it doesn’t support custom themes or themes available in the Chrome addons Store like Vivaldi does. Attempting to do that triggers “Themes are not supported” message in An error has occurred dialog, this is also the case with another Chromium-based browser such as Opera when it comes to Chromium themes.

Do note till now you can add themes to Microsoft Edge via a couple of methods, where the first one involves obtaining the theme CRX file and dropping onto edge://extensions page, where the second method needs running Edge with “--enable-features=msAllowThemeInstallationFromChromeStore” command-line switch. Both methods are complicated when it comes to theme uninstall as there is no UI available in Edge to do that. Now installing and uninstalling themes from Chrome Web Store in Edge gets a lot more easer.
1. Ensure you’re using latest Microsoft Edge Canary build 82.0.444.0 or higher
2. Visit edge://flags/#edge-allow-store-extension-themes
3. For “Allow installation of external Store themes” flag, select Enabled and restart the browser

4. After the relaunch, visit Chrome Web Store Themes page
5. Select any theme and click on Add to Chrome, this time the theme will be added to Edge without any error or warning dialogs
READ: Google releases 14 new themes for Chrome to the Web Store
How to restore Edge theme to default
1. To uninstall an installed theme in Edge browser, click on ellipsis icon and select Settings
2. Select Appearance and click on “Remove” opposite to “Custom theme” this reverts the Edge theme to default. You can now visit the Chrome Web Store and add another theme you like.

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