Theme header is part that consists of navbar branding and navigations (navigation and signin option). 

Theme core headers are located in the headers folder in the parent theme. You can set any header with JM Layout plugin help. Just select the needed header from the available list. By default, only parent headers are listed. If your theme doesn't support JML (no parent theme), then look for headers folder directly in theme folder.

The selected header is inserted in {---HEADER---} in your theme.html. If you need to customize your own header, you can:

- created headers folder in your theme and add there your own header file (copy existing and customize it). With a parent theme, you can use the same header name. This file will be added to the available header list in JM Layout plugin.

- or you can replace {---HEADER---} with your own code. Then only one header will be available. 

- or you can delete this shortcode from theme.html and manually put the needed header code in your layouts file.