"Dark Theme for Google™" extension brings a customizable dark theme to some of the Google services such as Google Search and Google Images. The extension supports all localized Google domains.
Note that you can customize many colors including background and link colors on the options page of the extension if you don't like the current configuration.
1. You can define a time period for the extension to automatically activate the dark theme
2. You can append your custom CSS rules to the default dark theme from the options page
This extension is an independent open-source project and is not related to Google Inc in anyways.