Add Olivero fallbacks to Gin variables in overlay styling

Problem/Motivation

The use of Gin variables to style the duration overlay was added in #3485266: Use Gin colors in duration overlay, but the fallbacks for the colors just use shades of grey. We can anticipate that some users may not have access to the admin theme but are able to create content. Though we can't anticipate what colors might be available in custom themes, we can look for Olivero-provided CSS variables.

Proposed resolution

Update the CSS to use the Olivero variables (if available) before using the shades of grey.