Use Bootstrap colors for html output

[davbeek]

For html output, we have hard coded the colors in the html template. We should replace these colors by the colors defined in Bootstrap colors.

For Excel, we cannot directly use the Bootstrap colors, but I expect the Excel output to become less important anyway, because Boottrap html has several advantages over Excel:

1. Compatible with all major platforms, unlike Excel which is available on Linux only via Libre Office.
2. Loads much faster
3. More user friendly

Furthermore, the html output functionality can be extended via many extensions to match the Excel functionality. Therefore, we can keep the Excel output unchanged.