In my opinion, one of the best ways to learn about wordpress theme development is to take one of the many clean and nicely layed out free themes and start customizing them yourself.
With this hands on approach with a working model, you have the ability to see for yourself what is happening to the theme with each of the changes that you make. The guesswork becomes less guessy when you have an established functionable framework to start with, and then the tweaking teaching takes you through each step of the personalization process.
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