What is the purpose of CSS custom properties (variables), e.g. "--main-color: #336699;" used as "color: var(--main-color);"?

A They are processed only by preprocessors like Sass, not by browsers directly B They allow defining reusable values that can be referenced throughout a stylesheet (and updated dynamically via JavaScript or media queries), reducing repetition and enabling theming C They can only store numeric values D They must be declared inside a <script> tag