I can suggest you the following solution for a dark theme when using Evernote from a Web browser. There is a "theme manager" called Stylish, which provides a way to install themes for different websites (not just Evernote, so you can use it for other "white" sites).
Steps for installation:
1. Install Stylish:
- for Chrome - https://chrome.google.com/webstore/detail/stylish-custom-themes-for/fjnbnpbmkenffdnngjfgmeleoegfcffe?hl=en
- for Mozilla Firefox - https://addons.mozilla.org/en-US/firefox/addon/stylish/
2.Install a dark theme:
- a dark theme for all web sites - https://userstyles.org/styles/47391/midnight-surfing-alternative
- dark themes for Evernote:
1. https://userstyles.org/styles/113637/evernote-dark-theme
2. https://userstyles.org/styles/114069/dark-evernote
