As a starter for you, try placing the below into your userchrome.css file.
/* For styling the entire Joplin app (except the rendered Markdown, which is defined in `userstyle.css`) */
/* CSS for the Dark theme to make the CodeMirror editor mimic the scheme of the old ACE editor */
span.cm-header {
color: #CF6A4C;
font-size: inherit !important;
}
span.cm-link-text, span.cm-strong, span.cm-em {
color: #8F9D6A !important;
font-style: normal;
font-weight: normal;
}
span.cm-variable-2, span.cm-hr {
color: #F9EE98 !important;
}
span.cm-meta, span.cm-property {
color: #7587A6 !important;
}
span.cm-string.cm-url {
color: #DDDDDD !important;
}
span.cm-comment {
color: #DAD085 !important;
}
It's not perfect but is does mimic the old ACE editor. I have only tried it for the default notes and I am not even going to think about trying to modify the way CodeMirror detects and displays real code in fenced code blocks!!
This screenshot is actually of Joplin v1.1.4
