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.
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.