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CSS Variables are values we define to be re-used throughout a CSS document. CSS variables are set in one place and referenced at many places like you will set a variable in other programming languages. A variable in CSS is defined by starting it with two dashes, followed by the name of the variable. The var keyword that we use to access a custom variable takes two parameters. The first one is the name the property to be substituted, the second is a fallback value. There is a significant reduction in the number of line in code that was written.