Setting Selected property of Row in TreeView dynamically in Gtk3 (python) - python-3.x

Question

This is the same problem as in my previous question, but I moved to python3/gtk3 to be able to use a css for setting the basic properties.
From the python file:
self.w = Gtk.Window()
self.w.set_name("App")
I can use a css:
#App GtkTreeView row:selected {
border-color: #000000;
border-top-width: 1px;
border-bottom-width: 1px;
color: #000;
}
And easily permanently change the style of the selection. To me this means that I should be able to dynamically get access to the row-object and its style where I could set the bg for the Gtk.StateFlags.SELECTED.
I've tried a bunch of weird ways, e.g (where bg_color is a Gdk.Color that works fine for e.g. changing the style of a Label outside the TreeView).
style=self.treeview.get_style_context()
col = style.get_background_color(Gtk.StateFlags.SELECTED)
col.alpha = 1.0
col.blue = bg_color.blue
col.red = bg_color.red
col.green = bg_color.green
Or:
style = self.treeview.get_style().copy()
style.bg[Gtk.StateFlags.SELECTED] = bg_color
self.treeview.set_style(style)
(produces error: style.bg[Gtk.StateFlags.SELECTED] = bg_color
IndexError: list assignment index out of range)
etcetera...
So please, how do I find the way to dynamically change the selection effect depending on the normal-color of the row? Or in other words, how do I find my way to the object that actually holds the style-setting for the selection?

Answer 1

I had one last idea about how it could be done after posting which actually ended up working:
Reloading the css dynamically:
In the css I added a row leaving the value for the background open to dynamic substitution:
#App GtkTreeView row:selected {
border-color: #400;
border-top-width: 2px;
border-bottom-width: 2px;
background: {0};
color: #000;
}
Then I loaded the css in python:
screen = Gdk.Screen.get_default()
self._css_provider = Gtk.CssProvider()
css = open("notify_stack.css", 'rb')
self._css = css.read()
css.close()
self._css_from = bytes("{0}".encode("utf8"))
self._css_provider.load_from_data(self._css.replace(
self._css_from,
bytes("#fff".encode("utf8"))))
context = Gtk.StyleContext()
context.add_provider_for_screen(screen, self._css_provider,
Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION)
Later in the callback for when a row is selected I put this row (data is my ListStore):
self._css_provider.load_from_data(
self._css.replace(self._css_from,
bytes(data[rows[0]][self.BG_COLOR].encode("utf8"))))
It feels really brute, there must be a nicer way, but hey it actually worked.