This doesn't quite work, cannot get to unset on exit.
I set 2 colour variables:
let vColHighlight = 'rgb(193,217,255)';
let vColNoHighlight = 'rgb(132,189,220)';
For a sheet, I then use a trigger like this:
Set another variable vViewMode
Set an OnActivate Trigger: set variable vViewMode to be Dashboard
Then in General->Custom Colours, I set the colour
=if(vViewMode = 'Dashboard', $(vColHighlight), $(vColNoHighlight))
However, I can't get the OnLeave Trigger where I set vViewMode to be 0, '' for it to change to vColNoHighlight,
I think this might be a good option: https://community.qlik.com/t5/QlikView-App-Development/Setting-active-tab-color/td-p/277965
