‘На тему’ is normally a brief description of what a conference is about, whereas ‘под девизом’ stands for ‘under the motto’, and it is usually a catchy phrase or slogan that should attract attention, stir up certain feelings, demonstrate allegiance etc,. So, ‘тема’ is more about reason and ‘девиз’ ferments emotions.

Eg. Сорок вторая научно-практическая конференция на тему “Применение ядохимикатов в сельском хозяйстве” прошла под девизом “Крысам – бой!”.