UI: no enough difference between pushed and unpushed state for buttons

[ldmpub]

Example on the "route [EN] / trajet [FR]" button:

Unpushed / inactive Pop-up

Pushed / Pop-up enabled

Issues:

• User may have pressed button by error and do not understand how to remove the popup to read top of message which is actually hidden
• when pressed, the button keeps this state when opening other mails => not sure if it's the wanted behaviour (note: personnaly I don't mind because now I have learned to press / unpress buttons 👎 ).

Running:

• Thunderbird 52.9.1
• Extension 3.1.8
• Windows 10