They're not explicitly asked for, but everywhere I've worked that's tested them has found that they improve conversion rates.

+1. users don't ask such things. they can't formulate them in writing (unless they are designers, artists, visual editors, UX experts themselves) and don't have time for that

do your follow any design guideline? what's your design principles?

I recommend picking some (e.g. Apple: https://developer.apple.com/design/human-interface-guideline...), and try to match that. there has been lots of though put into it, good to reuse before you came up with your own.