Hwy2015 developed a good one with 25 video shortcuts for Helix. It has been modified by others for Krypton. Hoping to see it modified for Leia. Fingers crossed. Most users I talk to do not like Estuary skin.
Many 17 skins will work fine with 18 if you just edit the addons.xml so it doesn't block it from being used, I doubt many developers will officially sign off on 18 skins until it officially replaces 17.x
That said, before you attempt to force a skin on 18, best to do a backup of the Kodi install so you can restore if it buggers it up completely...
Anyway download the skin zip or edit it in the userdata folder of your device... You want to edit the addons.xml for the skin only so find that file, open it with something like notepad and look for a line like this...
<import addon="xbmc.gui" version="5.12.0"/>
<import addon="xbmc.gui" version="5.13.0"/>
And then test the skin... But, don't blame me if it buggers up your install and requires a restore