What is QMenuBar? Is this a part of Qt Creator or something? And which file would it be to be modified, in the bitcoin source code for example?
QMenuBar is the Qt widget that draws the long menu strips at the top of the screen and also the entries in it (see above image).
You place a line like this in the same file, right between the <property> tag like this:
<property name="styleSheet">
<string notr="true">QMenuBar { background-color: color:#FFFFFF; }</string>
<string notr="true">QWindow { background-color: color:#FFFFFF; }</string>
</property>
QMenuBar class documentation for anyone interested in reading