I posted about this on the forum: https://forum.xfce.org/viewtopic.php?pid=52035 User ToZ reported that many themes use pixbuf to draw bars, and it seems that CPUgraph doesn't store the bar colour information in a way that pixbuf will read. A user workaround was also provided that might be useable as a fix here?
Andre Miranda referenced this bugreport in commit 739df2586d5d413e0a0e94d7a22018d1175f02ca Use css to change bar colors (Bug #15186) https://git.xfce.org/panel-plugins/xfce4-cpugraph-plugin/commit?id=739df2586d5d413e0a0e94d7a22018d1175f02ca