Dear @varina
The Row gutters are Margin percentages that are applied to each row - If your 'Row Gutter' is set to 5% then each row will have a 'margin-bottom' of 5%.
This unfortunately means that a row gutter is not a solid 'area' that can have a background colour applied to it.
https://www.w3schools.com/css/css_margin.asp
However from a design point of view there may be ways that you can trick this, one option being a 'border'
https://www.w3schools.com/css/css_border.asp
Here in this example i have set the row gutter to 0% but am using the border property to create a divide between rows - but the nice thing about a border is i can set a colour:
Screen Shot 2021-02-27 at 12.48.09 PM.png
To make things easier - in the Gridder layout i right-clicked the row and gave it a class using - Edit HTML class & ID.
I set the class to 'border_row_one'.
With that i applied the Custom CSS:
You can use other CSS unit than 'px' like 'VH' (viewport height)
https://www.w3schools.com/cssref/css_units.asp
For more information on using Custom CSS within Lay Theme check out the documentation page:
https://laytheme.com/documentation.html#custom-css-styling
Hope this has helped point you in the right direction and have a wonderful day :)
Richard