To access optin container options click on arrow button located at middle right side of optin editor.
It will open the options panel.
It will open container options which have container body style options, Global Styles, Custom CSS & Custom JS.
In Container Body Styles you can set container background options like background color, Image, Background Gradients etc and also set an overlay color on top of background image or color.
Global Font Styles
In Global Styles you can define default font options for your Landing Page so if you don't specify the font family & font size in widget options these will be used it reduces the work when you want to use same font options across your page.
Custom styling CSS without <style> tags. Added In head section of your landing page.
Custom JS without <script> tags, in case if you want to add some tracking codes or integrations specific to current landing page. If you want to add tracking code to all landing pages learn about it here : How to Add Tracking Scripts In Landing Pages.