Yes, there is a library. It's called Topcoat. It's owned by Adobe, but I don't know if they're still maintaining it. If you go to the Demo link, you will see all the customized HTML items. The library is a little limited, but it will give you a great start.
You can change the icons through your manifest. Just add the <Icons> tag under your <UI> tag.
<UI> <Type>Panel</Type> <Menu>PanelName</Menu> <Geometry> <Size> <Width>400</Width> <Height>400</Height> </Size> <MinSize> <Width>300</Width> <Height>300</Height> </MinSize> <MaxSize> <Width>500</Width> <Height>500</Height> </MaxSize> </Geometry> <Icons> <Icon Type="Normal">./icons/Normal.png</Icon> <Icon Type="RollOver">./icons/Rollover.png</Icon> <Icon Type="Disabled">./icons/Disabled.png</Icon> <Icon Type="DarkNormal">./icons/DarkNormal.png</Icon> <Icon Type="DarkRollOver">./icons/DarkRollover.png</Icon> </Icons> </UI>
I would try look into: c:\Program Files\Adobe\Adobe Photoshop CC 2017\Required\CEP\extensions\com.adobe.photoshop.crema\PSPanel\css\
I am not sure what exactly is .crema purpose, but It looks a bit like Plugin Template
Interesting, thanks for sharing Jarda! I'll have to wait until we get CC 2017 at work to look at this panel