The in-game menubar of FlightGear is located at the top of the screen.
The menubar is available in various styles. Currently there are two styles (both are shown below) packaged with FlightGear. The blue/black one is standard, but most developers use the grey/white one. You could switch between styles by pressing Shift + F10.
Creating your own
It's very easy to make your own designs. Just copy one of the files that you find in data/gui/styles and change it to your needs. Then, edit the follwing code in the data/preferences.xml file to fit your style. You could simply add a line like <style n="0" include="gui/styles/anthrax.xml"/>. The current-style tag specifies the style that should be used when FlightGear loads.
<gui> <current-style type="int" userarchive="y">1</current-style> <style n="1" include="gui/styles/classic.xml"/> <style n="0" include="gui/styles/anthrax.xml"/> <devel-widgets type="bool" userarchive="y">false</devel-widgets> </gui>
Aircraft dependent items
To add a menu for a specific plane only you have to enhance the planes -set.xml file. Add the follow lines (example for the livery selection dialog in a Boeing 787), or any item that you want to show, above the </sim> tag. You can add as many items and menus as you like, but keep the menubar as small as possible, the larger the outside view will be.
<menubar> <default> <menu n="10"> <label>Boeing 787</label> <enabled type="bool">true</enabled> <item> <label>Select Livery</label> <binding> <command>nasal</command> <script>aircraft.livery.dialog.open()</script> </binding> </item> </menu> </default> </menubar>