Slight improvement to signature look&feel

Use black for text color and border (instead of default red)
Also force the date to have timezone info, i.e.
  2021-04-13T15:32:29+02:00
instead of
  2021-04-13T15:32:29
4 jobs for !406 with work/aacid/slight_signature_lf in 26 minutes and 18 seconds (queued for 129 minutes and 49 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #100566
build_clazy_clang_tidy

00:10:49

passed #100565
build_ubuntu_20_04

00:06:13

passed #100567
clang_format

00:01:14

passed #100564
linux kf5-qt5 qt5.15

00:08:00

45.0%