updated dark theme screenshot + added one for light theme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
updated readme with screenshot for light theme

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
added example on how to change themes

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>