[BUGFIX] Allow overflow in default modal windows

The color picker is used in modal windows but may gets cutted as
the modal has an `overflow: hidden` definition. This definition
is moved to `.modal-dark .modal-content` as the definition
is essential for the image cropper.

Resolves: #77441
Releases: master
Change-Id: If24512a0ff37f0483dedaff527578e9e538f471f
Reviewed-on: https://review.typo3.org/49422


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Florian Steller <florian-steller@outlook.de>
Tested-by: Florian Steller <florian-steller@outlook.de>
Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>