Hi, as you have not given any example, explaining some common mistakes while using CSS variables. 1) Incorrect Declaration CSS variables must begin with --. Without it, the variable won't work. Incorrect: :root { primary-color: #3498db; /* Missing -- before the variable name */ } Correct: :root { --primary-color: #3498db; } 2) Using Variables Outside of var() Variables should always be used within the var() function. Incorrect: .header { background-color: --primary-color; /* Missing var() */ } Correct: .header { background-color: var(--primary-color); } 3) Undefined Variables If a variable is not defined, it will result in an invalid property value. Incorrect: .footer { color: var(--undefined-color); /* --undefined-color is not defined anywhere */ } Correct (with fallback): .footer { color: var(--undefined-color, #000); /* Falls back to #000 */ } 4) Scope Limitations Declaring variables inside a selector limits their scope. Incorrect: .container { --local-color: #ff5733; } .text { color: var(--local-color); /* This won't work as --local-color is not defined globally */ } Correct: :root { --global-color: #ff5733; /* Defined globally */ } .text { color: var(--global-color); } Other than the above, browser caching or incorrect path to CSS file might be the problem. I hope this helps :)