Custom propertiesCustom properties are CSS properties prefixed with -- that set values you can reuse with the var() function. For example, you can set a --key-color property to reuse as border-color: var(--key-color). Also known as CSS variables.cssCSSwidely2017-04-052019-10-05https://drafts.csswg.org/css-variables-1/css-variableshttps://developer.mozilla.org/docs/Web/CSS/Reference/Values/varcss.properties.custom-propertycss.types.varhttps://nowyoucanuse.com/features--custom-properties.html