How i can change the Staff Icon

[Canapin (Coin-coin le Canapin)]

This it not how it works. $shield-alt-duotone is a variable meant to be used in your custom CSS. The first topic I linked earlier explains how to replace icons with new ones with an SVG file:

Replace Discourse's default SVG icons with custom icons in a theme

You can replace a Discourse’s default SVG icons individually or as a whole with your own custom SVG and override them within a theme or theme component. Step 1 - Create an SVG Spritesheet To get started, you must create an SVG Spritesheet. This can contain anything from a single additional custom SVG icon up to an entire replacement set of hundreds. The spritesheet should be saved as an SVG file. In principle, you are nesting the <svg> tag contents from the original SVG icon file into <symbo…