Hi bhavesh,
This is just a warning and should not prevent you from submitting your theme to Themeforest as it is the case for all frameworks as there is not much we can do about that. In fact, there is another discussion here about that and Foodist made it to ThemeForest as you can see here.
Happy coding,
Check out WordPress themes for your blog from https://www.sktthemes.org/product-category/blog-wordpress-themes/
For creating WordPress theme, you should have hands-on-html or Web developing. This is the most important part of designing website (Web designing). This makes one more easy to create a Web page or a theme for WordPress.