Implement tests/ and code coverage

[jorgepiloto]

Description of the feature

This package lacks tests and coverage. However, all the logic source code is located in the __init__.py file. This template inherits from the PyData theme, so most of the logic is already tested there.

Also, it looks like there is a "sphinx.testing" package but it lacks from documentation, see issue sphinx-doc/sphinx#7008.

Steps for implementing the feature

I guess we could take a look at PyData theme and RTD theme to explore how a Sphinx theme is tested.

Useful links and references

[Revathyvenugopal162]

Pinging here issue #256 as we need adapt test for different screen resolutions.