Munk WordPress Theme comes with an inbuilt sticky header that you can enable from the Customizer of your site.
Navigate to Dashboard >> Appearance >> Customize >> Header >> Sticky Header
Enable the Sticky Header by changing the toggle. By default the sticky header is disabled on the theme.
Sticky Header Padding
If you would like to adjust the top and bottom padding of sticky header, you can do so using the control given here. By Default the padding is set to 5px. You can increase or decrease it as per your liking.
Sticky Header Logo Size
This option lets you reduce or increase the logo size for sticky header. On a Sticky Header the logo are usually smaller than the primary header. You can adjust the logo size to your liking.
Bottom Border Size and Color
You can also add or remove border size of the sticky header. Set the border size to 0px to disable border completely. Below that, you can choose the bottom border color for the Sticky Header.