¿Cómo puedo cambiar los iconos de Discourse por otros?

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No puedes simplemente cambiar de font-awesome a otro conjunto, necesitarás cargar tus nuevos iconos y reemplazar los actuales uno por uno.

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 <symbol> …