Customize display styles used for syntax highlighting of .languageGrammars?

Is there any way of customising the syntax-highlighting styles for TaskPaper, Markdown etc ?

( The ‘TaskPaper’ languageGrammar displays all bulleted items and unbulleded notes in italic, which, I find, slightly reduces the general legibility of the files.

It would be good to have the option of switching off the (items | notes) -> italic setting, and perhaps also changing the green color used by default for for the styling of Taskpaper projects ).