You can use the development tools to find the css element names.
See this post.
I haven’t modified my userchrome.css but if you want to change the colour, weight etc. of all the headers in the Markdown (#
, ##
, ###
etc.) it looks like you would need to add something like:
.ace_heading {
color: red;
font-weight: bold;
}
to your userchrome.css.