Improve the coloring scheme

[markomi]

Is your feature request related to a problem? Please describe.
Improve the coloring scheme.

Describe the solution you'd like
Unify the coloring scheme, e.g. the navbar and sidebar-toggle have a different color than the header in the original app. To achieve this, use a css sheet.

Describe alternatives you've considered
none

Additional context
As discussed here: secuTrialRshiny issue 3