On your searx site of choice (see list of supported sites on github repository) the theme has to be set to oscar and the style to Logicodev in the preferences to make this userstyle work.

repository on github: https://github.com/hubisan/searx-userstyle-dark-theme. Contributions are welcome.

changes 2017-03-13: added missing urls, added css styles to improve initial view (needed if installed with firefox directly > see github repo on how to do this)
changes 2017-02-01: added some !important
changes 2017-01-31: navbar darker, visited style, modal-close button more padding for mobile, panel darker, categories not full width on biggest screen (flex-grow 0)
changes 2017-01-30: visited style, vim box styles
changes 2017-01-30: added new css styles to make the theme work with newest release (live on searx.me)