If you want to build WordPress themes and make them available to the public, you need to become familiar with the GNU General Public License WordPress uses. If you don't plan to distribute your work as a free or paid theme, you don't have to worry about the license.
While GPL is a free license for software, you can charge money for your theme. There are four freedoms associated with the GPL License; the software is free for any purpose, people are free to study it and change it and people are free to redistribute copies of your theme and the version they modified. Imagine if WordPress, the world's most popular CMS, wasn't free. If the GPL license wasn't free, WordPress couldn't fulfill their mission of making publishing democratic. Plugins are also have a GPL license.
The GPL license doesn't sound good for developers who make themes to sell, but they can get money for updates and support. Additionally, it's only the PHP under the GPL license. The developer owns their artwork and CSS and can license them under more restrictive terms or they can apply the GPL license to them. Fortunately, many developers create themes publishers can use for free or for an affordable one-time fee.
If you're creating a WordPress blog, you'll find hundreds, if not thousands, of free templates for every niche. If you have a WordPress blog, you know how important it is to find the right theme. While premium themes and plugins are on Github, most people are happy to pay for them and get the support they need. Many people don't understand how GPL works either; they are happy to pay $75 or so for a theme that will give their business a professional online presence. Considering what a firm charges for a custom website, a customizable premium theme is a bargain. For more information click here https://alx.media/category/free/.