kcm: Fix Text not readable when using a Breeze Dark
…and generally port it to something more appropriate for a KCM.
Test plan if you don't have any Thunderbolt docks:
$ cd ~/kde/build/plasma-thunderbolt/bin
$ ./boltfakeserver --cfg ~/kde/src/plasma-thunderbolt/autotests/data/default.json &
$ KBOLT_FAKE=1 systemsettings kcm_bolt