Font weight is wrongly applied to gtk3 panel plugin context menues.
This applies to all gtk3 panel plugins in xfce4, but since there a not many, I filed this against power manager.
 
Steps to reproduce:
1. Use a theme with bold panel fonts (eg gtk theme Arc)
2. Right click on power-manager plugin.

Expected:
Normal font weight in the context menu, which would match the gtk2 behavior.

Actual result:
Bold font weight in the contect menu.

Further information:
The problem seems to result from the wrong css class used for theming the contect menu. As seen in the following snipplet from the gtk theme, .panel .menu should have been used for theming instead of .panel itself, resulting in unexpected behavior in context menues.

Snipplet from the arc gtk theme:
.panel {
  background-color: transparent;
  color: white;
  font-weight: bold;
  text-shadow: 0 1px 2px rgba(0, 0, 0, 0.6);
  -gtk-icon-shadow: 0 1px 2px rgba(0, 0, 0, 0.6); }
  .panel-shadow {
    background-image: none;
    background-color: transparent; }
  .panel .menu {
    box-shadow: none; }
    .panel .menu .menuitem {
      font-weight: normal;
      text-shadow: none;
      -gtk-icon-shadow: none; }
    .panel .menu .window-frame.menu.csd,
    .panel .menu .window-frame.popup.csd {
      box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.2), 0 10px 20px rgba(0, 0, 0, 0.19), 0 6px 6px rgba(0, 0, 0, 0.23); }
  .panel .menubar > .menuitem {
    padding: 3px 6px; }
    .panel .menubar > .menuitem:hover {
      background-color: transparent; }
  .panel .window-frame.menu.csd,
  .panel .window-frame.popup.csd {
    box-shadow: none; }