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 in 5 steps
Follow the three step images and you can add the project script you can find in Project Settings to Tumblr.
Step 1: Edit the theme in your Tumblr Dashboard
Step 2: Edit the theme HTML
Step 3: Find the Project Code in Convert Experiments
Step 4: Copy Basic Code from Convert Experiment
Step 5: Paste the Convert Experiments Code in Tumblr
Save your changes and you can start A/B testing on Tumblr.