Tweak misterioso header line background color

* etc/themes/misterioso-theme.el (class): Make the header line
background darker so that cedet function names and parameters are
legible (bug#33061).
1 job for master in 84 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #2494
test-all

01:24:20