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If you don't have Stylish (or Stylus) extension already installed, you can't install anything currently because the official Stylish extension is removed from extensions gallery both in Chrome and Firefox due to its issues with privacy.
Simply install Stylus, then you'll be able to install the themes from userstyles.org. Alternatively, if you have Greasemonkey or Tampermonkey or Violentmonkey extension installed you can use Install style as userscript link on the style page.
Install style as userscript