Mains cables will not make a difference unless they are damaged, which in that case replace them as straight away.
There are many claims about how mains cables improve the sound however, none have ever been scientifically proven or verified, so take any claims with a pinch of salt. (Do a double blind test if you wish to try for yourself?)
When it comes to looks (Some people like to have their cables on show) the standard cables provided by manufactures look horrible, so in this case no harm in getting something that looks better, but they will not improve the sound in any way.
Bill