To add this section, please follow the below steps:
From your Shopify admin, go to Online Store -> Themes.
Find the theme that you want to edit, and then click Customize.
Scroll down to Popup group at the bottom, Click Add section and choose "Newsletter popup".
Show fixed toolbar
Enable/disable checkbox
Heading
Add text to display as a section heading (Leave the field blank to hide the heading)
Text (Color)
Set the text color for this block
Background
Set the background color for this block (If a background gradient is set, the section's background will be set according to the background gradient setting)
Background gradient
Set the background gradient for this block
Show Again After (Days)
Determine the time span between each automatical showing of the popup
Delay Time (Seconds)
Determine the delay time for the popup to automatically shows up
Image
Set up an image (1000 x 600px .jpg recommended)
Color scheme
Select a color scheme to set the text and background colors for the section