Updating homepage class

Following suggestion in #1954 (comment)

Author: erikvansebille

docs/index.rst

@@ -5,6 +5,7 @@ Parcels documentation
 Welcome to the documentation of Parcels. **Parcels** (Probably A Really Computationally Efficient Lagrangian Simulator) is a set of Python classes and methods to create customisable particle tracking simulations using output from Ocean Circulation models. Parcels can be used to track passive and active particulates such as water, plankton, `plastic <http://www.topios.org/>`_ and `fish <https://github.com/Jacketless/IKAMOANA>`_.
 
 .. figure:: _static/homepage.gif
+   :class: dark-light
 
 *Animation of virtual particles carried by ocean surface flow in the global oceans. The particles are advected with Parcels in data from the* `NEMO Ocean Model <https://www.nemo-ocean.eu/>`_.