This theme is a gray/blue mix theme that is sometimes referred to as "Cyan-Light". I enjoy the color of gray chosen as it easier on the eyes and it is not dark which some people may not like. It is hard to mix in true cyan as it would be too bright, so I opted for a cooler cyan-blue.
To install, simply navigate to your vault. Inside, drag and drop the cyan-light theme.css file into: \.obsidian\themes
Then, open the Obsidian application and go to Settings => Appearence => Theme and select cyan-light theme
Obsidian.md is a really cool note taking and mind-mapping software that is powered through the use of markdown.
This software also allows you to modify your CSS files to give it a custom look.
Credit to maker of 80s Neon theme for Obsidian which helped me to get started with finding classes and seeing how Obsidian worked.