Creating custom skins really depends on your programming skills and familiarity with Linux. If you search on the Kodi wiki you will find plenty of info.
To start off, Kodi recommends modding an existing skin - perfectly permissible under the terms of an open source licence. Forum members have started to mod the Slice skin to use other fonts, for example.
I have been fiddling with a modded skin and am close (!) to being able to incorporate different themes. What's most important is to copy a skin from Slice onto a computer - that way you're not going to mess up your Slice!