A/B or multivariate testing on you Tumblr page is fairly easy, follow the steps below to install the Convert Experiment script in you Tumblr theme.
Edit Tumblr Theme
Follow the steps and you will be able to add the project script you can find in Project Settings to Tumblr.
Click on the Account icon on the top-right corner. Select Edit Appearance. Then click on Edit Theme button
Edit the theme HTML
Find the Project Code in Convert Experiments
Copy Basic Code from Convert Experiment
Paste the Convert Experiments Code in Tumblr
Save your changes and you can start A/B testing on Tumblr.