There is a wiki page on this forum that lists some css customisations. The first one uses userchrome.css to remove almost all formatting in the editor pane.
Plain editor (no color, no font changes)
userchrome.css
Joplin version tested on: 1.1.4
div.CodeMirror-lines * {
font-size: inherit !important;
color: black !important;
font-weight: normal !important;
text-decoration: none !important;
}
Example