Here’s an excellent guide to get you up to speed:
And in the theme.css, it looks like this here:
The coloring change may also be the block quote in the callout. Callouts tend to layer colors on top of each other so a plain block quote background, already on a colored background, could look different if inside a callout.
If you paste an example of what’s written in your note and any extra CSS (custom callouts, etc) you’ve added, someone can probably have a look.
Also, this theme is very pleasant to look at. 