To define the Sticky(fixed) Header bar, please go to ART Theme > Theme Options > Header.

There are 3 options:
- None: there is not any sticky effect.
- Always Sticky: the header will be sticky when page scrolling.
- Sticky when scrolling back: the header will be sticky when page scrolling to top.
You could set height for sticky header bar. Please enter the integer number. It will be shown by the default size if they are empty.