Decorative shapes overlap section subheadings

[shyusu4]

Issue

Decorative shapes are currently overlapping with section subheadings.

Steps to reproduce Resize the viewport to 768px or less.

Suggested fix Make sure icons are always behind text elements in z-index layering or/and add a solid background according to theme behind the text.

Reference Wagtail.org | Accessibility audit spreadsheet

[thibaudcolas]

There’s been some improvements in #458 but I think there’s more work to do on this.