CSS is definitely for those who are already armed with the knowledge and expertise of using them. We do already give account holders the ability to change colours and fonts in ACCOUNT>PRESENTATION in a nice easy format though, so CSS isn't necessary (unless you really need to!)
but if that is not enough you can add your own CSS in ACCOUNT>GUEST EXPERIENCE.
(please note the warning above!)
Things you should take into account are the Red and Green colours on the guest view dashboard - we STRONGLY recommend keeping the colours clear (think Green for go and Red for NO! or similar) - Never using Greys of any sorts, as it can make your availability look sold out!
If you need further support on CSS, then this guide can help explain everything you need to know (but if you need to know this is probably not the setting for you!)
Whilst our Support team are amazing, we are unable to help where a custom CSS has been used! as this is an unsupported feature.