At the moment, FireMonkey uses CodeMirror 5 for the code editor.
CodeMirror 5 uses CSSLint & JSHint for linting.

Unfortunately, CSSLint has been abandoned (last release Sep 20, 2016), therefore it doesn't support newer CSS implementations.
JSHint hasn't been updated for a while.

I have been considering the update of the editor, but haven't been able to find a guide.

Note

There is a major consideration with bundling of software. In FireMonkey, some CodeMirror files have been updated to add more specific features. Such updates will not be possible with bundling packages.

CodeMirror 6

CodeMirror 6 - codemirror/dev

• Request Bundling Guide
• Is there a step-by-step guide to pack CodeMirror to use in a browser extension?
• Question about interactive-examples.mdn.mozilla.net

Monaco Editor

Monaco Editor

• Is there a pure JavaScript online version of Monaco Editor?
• How to use Monaco Editor in a browser extension?
• Webpack error with Monaco Editor

Ace

Ace - Ace

• Minimal embed with JavaScript & CSS

Webpack

• Webpack error with CSS loader
• Webpack CSS loader issue

See also:

• Support linting for CSS nesting
• FireMonkey: CSS error checker is faulty for more complex CSS rules. Option to turn it off?