Thanks to a typo, I stumbled on the answer. Thanks, wobbly fingers!

my $theming = "textview text {\nbackground-color: #000000;\ncolor: #FF
+0000;\n}";
$provider->load_from_data($theming);

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In reply to Re^2: Gtk3 and CSS styles by Anonymous Monk
in thread Gtk3 and CSS styles by Anonymous Monk

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