Step 1: Set Visual Studio Color Theme to ‘Dark’
Tools → Options → General → Color Scheme → Dark
Step 2: Import the downloaded settings file
Download Monokai-VS2019-CSharp-ChrisFrewin-0xothik.vssettings from this repository (from "Releases" category, for example) and import settings from this file:
Tools → Import and Export Settings → Import selected enviroment settings → [Do backup of current settings, if you need] → Browse → [Open downloaded Monokai-VS2019-CSharp-ChrisFrewin-0xothik.vssettings] → Finish
Theme base — by Monokai's original creators
Visual Studio's version — Chris Frewin, Michael Armes and 0xotHik