Tweak Neo Dark with custom CSS

# Problem
Every time I apply a custom CSS, it uses the light theme as a base theme. I want to use the default Neo Dark theme and apply my custom CSS tweaks there.

# Solution
Create a publicly available note with Neo Dark theme, so that I could include it to my notes, and apply additional custom CSS tweaks.

## Alternative solution
Don't always use light theme as default theme for custom CSS. Just apply the custom CSS to whatever is chosen by the user by default.