Custom Header Sticky across all community pages

You can add your header code into one component and add that to page wrapper in to Page Header of the community style.

To edit a wrapper for a custom skin:

1. Go to Studio > Community Style. 
2. Select Community Skins from the Work with drop-down. 
3. Select the skin you want to edit.
4. Click the Wrapper tab. 
5. Enter the code you want to add in the wrapper sections you want to add.
6. Click Add Notes to annotate your changes for each section.
7. (Optional) If you want to revert to the default for each section, click Use Default.
8. Click Save.

Here is the documentation of that.