On Google Chrome

How to install CSS Pro on Chrome:

Step 1. Download and unzip the "CSS Pro.zip" file from your Gumroad's receipt or library.
Inside it, you'll find the files README, Chrome.url, Firefox.xpi, and a .dmg file:
Step 2. Open the "Chrome.url" file to install the extension on your Chrome browser. It's going to take you to the extension on the Chrome Webstore.
Step 3. Click on "Add to Chrome" to download the extension.
Click on the blue button "Add to Chrome" on the right side to download the extension
Now that you have the extension installed, it's time to activate it with a license key, which you've got in your receipt, but can also be retrieved here if you forgot/lost it.
Step 4. Open any website on a new tab, for example, Google, or your own website.
Step 5. Right-click anywhere on the page and select "Inspect with CSS Pro" on the context menu.
A modal (popup) will appear asking for the license.
Step 6. Input your license key and validate it by clicking on "Check license key":
🎉 Done! Your license is now activated, and you can use CSS Pro anywhere else.
The extension will not ask for your license again (unless deactivated or disabled).
To manage your license activations in the future, use the My License page.

🎁 Bonus:

You can also pin the extension on your Chrome's topbar by clicking on "Extensions" (puzzle icon):
Find CSS Pro and click "Pin":
Now you can just open CSS Pro by clicking on its icon on your Chrome's topbar.