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You can have a few themes only if a designer made it for you first. But you don’t always have one by hand, you want to add a dark theme to a default one. Using CSS Custom Properties, you will have to write a few cycles on SASS. The process is done in 3 steps: choose the colors, add a use case, and add a new theme. If there are no colors for the dark theme, you need to use a special CSS media inquiry for that purpose: prefers-color-scheme.