DOC-2498: `Forecolor` and `backcolor` toolbar buttons were not completely greyed out while in readonly mode.

[kemister85]

Ticket: DOC-2498

Site: Staging branch

Changes:

• Fix documentation for TINY-11313

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.

Review:

• [x] Documentation Team Lead has reviewed