[linux] fixed foreground colors for menubar
[linux] fixed foreground colors for menubar

Foreground colors used by menubar buttons should be consistent
with themes provided by the system. We use same foreground color
as menu items.

fixed #5007

nwjs/nw.js,nwjs/nw.js,nwjs/nw.js,nwjs/nw.js,nwjs/nw.js,nwjs/nw.js