This plugin allows extending and overriding the default theme used on the login and signup pages since Jenkins 2.128.
After installation, its settings are available in Manage Jenkins » Configure System » Login Page Theme
Two modes are supported and controlled by the option Use default theme: If checked, it will extend the default theme with additional content; otherwise, it will replace it entirely.
See the related announcement blog post for the new 2.128 login theme for technical background.
In very rare configurations (those distinguishing between Overall/Administer and Overall/RunScripts, and not allowing administrators to configure the Markup Formatter), this functionality could amount to a potential security issue. Only install this plugin, or make it available for installation, if you trust all your Jenkins administrators not to abuse these options.