theming: add guards to prevent overriding default monaco coloring

[vince-fugnitto]

Motivation:

#7351 (comment)

Description

There should be guards in place to prevent overriding monaco coloring.
Monaco defines it's own colors which are used across the application, and it is possible to override such colors which may cause theming issues. Guards should be put in place to prevent such overriding such as to discard updates and log them.

[akosyakov]

Maybe we can even do it generically and ignore overriding colors with error logging. One has to provide a custom theme if they want to have custom theming, not override colors.