[TASK] Improve visual identification of read only fields

To improve the visual identification, e.g. on screenshots, the text color
is lightened a bit.

Resolves: #71103
Releases: master
Change-Id: I33573a3d37fbc55f29e72dcb8e992cf6e994ed1d
Reviewed-on: https://review.typo3.org/44334


Reviewed-by: Sebastian Fischer <typo3@evoweb.de>
Tested-by: Sebastian Fischer <typo3@evoweb.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>