This is the best place to get free HTML Codes to start creating new Tumblr theme. All codes are free and 100% customizable. You can put it on your new Tumblr theme.
According to Tumblr, You must use third party widgets and codes as a optional. So here is the codes to make them optional.
Endless Scrolling: This HTML code makes your theme infinite scrolling. All most all theme have this code installed. You can convert your theme in infinite scrolling. Below is the installation tutorial.
Scroll to top Button: You can’t make it optional. However, It works great with infinite scrolling themes. If you are planning to use endless scrolling code, then i recommend you to use scroll to top button widget.
Background, Font, Color Changer: This HTML code will help your theme to upload custom background image with optional option. The most wanted option in any theme by Tumblr users.
Above are some most common widgets and HTML codes for your theme. You can get free basic Tumblr Theme code to make your own Tumblr theme.