Just had some fun creating my own "persona" (theme) for my Firefox browser.
This might be a good activity for your students. They need to create a long, narrow photo for the top and the bottom of the browser, and then upload it to the site. All the step-by-step directions and specifications are at the site (a good reading activity, and/or watch the tutorial video), and they can try it out before uploading. After the photo is accepted, they can share their personal favorites and/or make their theme public for others to use.
You will need to be able to download an add-on to Firefox before starting the process, so this may require the OK from your school administration or tech support. However, the download itself takes only a few seconds.