Hi Mehtaz,
I realise I can use custom CSS - that's not the issue here.
The issue is that the button group addon has a field for background colours of the buttons, but this isn't working - it's setting the button background colour to 'transparent' regardless of the colour you add to the background field.
This is a bug in the addon that I thought you might like to let the Devs know so they can fix.
Custom CSS isn't a great solution for clients as they don't know CSS.
It's also worth noting that we shouldn't have to fix bugs with custom CSS - they should be picking up the colours within the field - not setting it to transparent.