Porting Gtk2 styles to Gtk3

[flo xfce gmx-topmail de]

I am struggling how to translate operators of the form
color = &style->light[GTK_STATE_PRELIGHT];
into proper Gtk3. The documentation says:
"One notable difference is the reduction from fg/bg/text/base colors to
only foreground/background, in exchange the widget is able to render its
various elements with different CSS classes, which can be themed
independently."
First question would be: What do the different colors stand for anyway?
What does light and dark mean when it comes to rendering? For example, I
am using the Arc Theme which does not provide .light or .dark classes
for different widgets. But I also don't want to provide an
application-specific CSS file because the application should follow the
system theme.
The second question would be how style properties fit into this as well.
Some colors are set explicitly via style properties, but those are also
deprecated. What would be the way to replace those?
I am looking for the recommended way/best practise of styling widgets,
not some hacks and workarounds. Can anybody help me?

Thanks and kind regards