Write some paragraphs. Mark one of it and create a section (Insert > Section). Set cursor into the section and call Format > Sections. Click on button "Options". The vertical tab bar of that dialog has an item "Indents". This icon is "paraspaceincrease". It is not suitable because it shows vertical arrows whereas the section indent settings make horizontal changes. I have looked through the cmd/32 folder of the Colibre icon-theme. I have not found an icon that fits exactly to this section property. Best one of the existing icons is "incrementindent", because the default is an indent of 0cm and someone that uses this property will do it because he wants to increase the indent. Actually the "Indents" tab does allow to change indent before and indent after. So an icon should show two arrows, I think. But it is a task for the designer community to create a suitable icon.