1) Default Style
2) Mobile Style
Many of my members visit my forum throughout the day and often are either on their PC or smart phone. They are complaining that it's a pain to have to switch styles as they go between their PC and mobile phone.
I tried having only the Default Style and code responsive css to tailor to the mobile phone, but that became very messy, so I prefer the simplicity of having two styles. The only short coming is having to switch styles all the time.