Created attachment 6887 Visual of the issue. On the left is xfce4-cpugraph-plugin using GTK2, on the right is xfce4-netload-plugin with the issue. Recent versions of GTK3-based panel plugins with meters (xfce4-battery-plugin 1.1.0, xfce4-wavelan-plugin 0.6.0, xfce4-netload-plugin 1.3.0) have had blank white backgrounds for the meters as opposed to using the background of the rest of the panel in a dark theme (or if the panel is set to a dark color manually). It looks distinctly out of place and is rather distracting. Attached is an image showing xfce4-cpugraph-plugin on the left using GTK2 and matching the background color of the panel, and the netload plugin on the right with the white backgrounds. It's done with the numix-frost theme, but I've verified it is not related to that theme by setting the panel to pure black in the panel config to similar results. Apologies if this is the wrong item to file the bug under, but this seems to be the root library used by all the plugins to get gtk2/gtk3.
Cant it be a theme issue ? Simon, any idea ?
If I change themes, it does go to a different color for the background of the bar in some cases (see the Adwaita theme for that), but it isn't matching in a lot of cases and doesn't follow at all if I set the toolbar to a fixed color.
This is a theme issue. I've recently fixed it in Greybird: https://github.com/shimmerproject/Greybird/commit/10929b1211b7a3377e1d8bcb2b2b37317b709d82 In theory we could include some code in the panel, but that would also break some themes...
This has to be fixed by themes. The issue you reported concretely was fixed in the Greybird theme, as mentioned above.