Facebook Glass Pane Theme
To apply this style, download the one of the following extensions, TamperMonkey, GreaseMonkey, and Code Injector. Copy the CSS code and paste to editor panel's. Then saved and inject/apply.
The userstyles.org website offers a convenient way to download and install themes. You can browse through various themes available on the website and apply the one you like by clicking the "Install" button.
This style: https://userstyles.org/styles/267372
