Coverage report refactoring

[Blebowski]

This MR tries to beautify the Code coverage HTML report style.

• Font is changed
• Table headers are unified
• Buttons are converted to table headers
• Mismatching widths are aligned
• Colors are slightly unified

There are following additional fixes and improvements:

• The rounding in the coverage report is fixed so that numbers like 99.9999 do get rounded down to 99.9, rather than to 100.0
• Average coverage result is printed to the command line - This is to allow grepping overall coverage number in CI pipelines.