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
