code: public static void setDefaultTooltipManager(final TooltipManager tooltipManager) {
          try {
              TooltipManager.getInstance();
              Reflection.setFieldValue(ClassReflection.getDeclaredField(TooltipManager.class, "instance"), null,
                      tooltipManager);
          } catch (final ReflectionException exception) {
              throw new GdxRuntimeException("Unable to set default tooltip manager.", exception);
          }
      }
docs: Since main tooltip manager instance cannot be changed globally with a regular setter, this method modifies it
  using reflection.
  
  @param tooltipManager will be returned on {@link TooltipManager#getInstance()} calls.
  @throws GdxRuntimeException if unable to change manager.