On the main page of your journal, click on "manage categories", then click on the category you want to delete, then you'll see an option to delete it. If you have plants in the category, you will have to delete all of them first or else you won't be given the option to delete the category. But if the category is empty, you should have a delete option.
Sounds like you're on the right page, you should see the first main heading that say Your Journal Summary, then the next main heading down says Main Menu, and the third thing in the list under Main Menu is Manage Categories.
Your Gardening Journal
Our journal system is a powerful (and complex) system for professional journaling of "whatever you want". Keep detailed lists of what's growing in your garden, how long it took for seeds to germinate, etc, etc.
Then I see
You now need to start your journal off by adding for first item. You do this by going to the category in which you want this item to be created. When you have selected your category, then click on "Add a new item".
Select the category from the list below:
Theme Master (0)
Theme Master is what I want to get rif of.
Oh, you haven't finished setting your journal up yet. For now, ignore the fact that you don't want that category and put a plant into it anyway (doesn't matter which one since you're going to have to delete it, just make something up). It's been ages since I set up my journal so I don't know what the rest of the steps are--I think once you add a plant then you're done, and once you've finished the setup part then you should be able to go to My Tools and Journal and see the page I'm describing.
Once you see the main page that I'm talking about, before you can delete the category, you'll have to go delete the plant that you added--to do this, scroll down a little farther on that main page to the section that says Access By Category, then click on your Theme Master category. This'll bring up a page that shows the plant that you added, and in the right hand column you should see a link that says delete. Once you've deleted the plant, then go back to the main page and you'll be able to delete the category as I described above.