Sharing a cute desktop background is inappropriate but it's only a picture on a background, misusing an internal security mechanism for your own pet activism when visiting a harmless website is a totally different tier of unprofessionalism.
Do you activism in your personal time. Don't use important company resources used to protect employees from harm to push your slogans. Regardless if those slogans are legally legitimate statements in a normal context, like handing out a leaflet on the edge of the property.