Consistent bg colour for group sum rows. Labels for 'None' groups.

https://www.openproject.org/work_packages/9835

Group sum rows were using the odd/even highlighting which was unwanted. Just highlighting them all now (assuming that's what we want).
'Sum for...' labels for 'none' groups didn't have a label here. I set the default to be '-' so that it matches the group header.
let's use a SASS variable for theming and better maintainability