


What are CSS Custom Properties (variables), and how do they improve maintainability and theming?
Jun 19, 2025 am 12:48 AMCSS variables improve style maintenance and theme management by defining reusable values. Its core advantages include: 1. Reduce duplication and improve consistency by centrally defining style values; 2. Support dynamic coverage to achieve simple and efficient multi-theme switching; 3. Avoid common pitfalls such as inconsistent naming, overuse, etc. Developers should define global variables in:root, and use scope and fallback values ??to enhance flexibility and readability, while dynamic adjustments are combined with JavaScript to improve user experience.
CSS Custom Properties, commonly known as CSS variables, are a feature in modern CSS that allows developers to define reusable values ??within a stylesheet. These values ??can be referenced and reused throughout the CSS codebase using the var()
function.
At their core, custom properties work similarly to variables in programming languages. You define them with a name (starting with two dashes, like --main-color
) and assign them a value that can be used anywhere else in your CSS.
They're not just about convenience—they significantly improve how we manage styles across large projects, especially when it comes to maintainability and theming.
Better Maintainability Through Centralized Values
One of the biggest advantages of using CSS variables is the ability to centralize style definitions. Before variables, if you wanted to use the same color or spacing value across multiple components, you'd have to repeat it every time.
With custom properties, you define those values ??once—usually at the top level, often on the :root
selector—and then reference them wherever needed:
:root { --primary-color: #007bff; --spacing-unit: 1rem; } .button { background-color: var(--primary-color); padding: var(--spacing-unit); }
Now, if you ever need to change the primary color or adjust spacing, you only have to update one line instead of searching through your entire file. This reduces repetition and lowers the risk of inconsistencies.
Also, since these variables are scoped to specific selectors, you can override them in different parts of the page if needed, giving you flexibility without sacrificing control.
Simplify Theming with Dynamic Overrides
Theming used to require either complex preprocessor logic (like SASS maps) or separate stylesheets for each theme. With CSS variables, switching themes become much simpler.
You can define a set of base variables for your default theme, and then override them conditionally—say, by applying a class to the <body>
element or using media queries:
:root { --bg-color: #ffffff; --text-color: #333333; } .dark-theme { --bg-color: #121212; --text-color: #f5f5f5; }
Then, in your HTML:
<body class="dark-theme">
This approach makes it easy to support dark mode, user-selected themes, or even seasonal variations without duplicating most of your styles. Just swap out the variable values, and everything built with them updates automatically.
It also plays well with JavaScript—you can dynamically change theme variables based on user preferences or interactions.
Avoid Common Pitfalls and Keep It Readable
While CSS variables are powerful, they come with a few gotchas:
- Variable names are case-sensitive , so
--MainColor
and--maincolor
are treated as different. - They cascade and inherit , just like regular CSS properties. That means where you define them matters.
- If a variable is undefined or misspelled inside
var()
, the browser uses the default value (if provided) or falls back to the property's initial value, which can lead to unexpected results.
To avoid confusion:
- Stick to a consistent naming convention (eg, kebab-case).
- Define global variables on
:root
unless you specifically need local scope. - Use fallbacks wisely:
color: var(--text-color, black);
Also, don't overdo it. Not every single value needs to be a variable. Reserve them for things that truly benefit from reusability—colors, fonts, spacing units, breakpoints.
Using CSS Custom Properties doesn't have to be complicated. When applied thoughtfully, they make styles easier to read, manage, and scale. Whether you're building a small site or a large design system, variables give you more control with less effort.
Basically that's it.
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