# It's Just JavaScript
Since immutable styles is just JavaScript it gets the niceties of CSS pre-processors for free. Leveraging everyday JavaScript enables variables, detached rulesets, *pre-processor* mixins, and theming with little to no extra cost.
## Variables
```jsx
const brandColor = 'plum';
// usage:
```
## Detached Rulesets
```jsx
const fontMedium = `
font-family: 'Open Sans', sans-serif;
font-weight: 600;
font-size: 2.1rem;
`;
// usage:
{ fontMedium }
```
## Pre-processor Mixins
You can create your own:
```jsx
const borderRadius = radius => (`
-webkit-border-radius: ${radius};
-moz-border-radius: ${radius};
border-radius: ${radius};
`);
// usage:
```
Or use a library such as [polished-styles](https://github.com/styled-components/polished):
```jsx
const { lighten } = require('polished');
// usage :
color: { lighten(0.2, 'dogerblue') };
```
*Not to be confused with* [*immutable mixins*](/immutable-styles/advancedconcepts/immutablemixins.md).
## Themes
```jsx
const ACTIVE_THEME = 'light';
const themes = {
light: {
appBackground: 'ghostwhite'
},
dark: {
appBackground: 'midnightblue'
}
}
// usage:
background: { themes[ACTIVE_THEME].appBackground };
```
## Server-side
Compiling immutable styles server-side – and of course infrastructure permitting – would enable futher functionality, such as generating CSS specific to:
* ⚗️ Experiments (AB testing)
* 🎌 Locale
* 📱 Platform
> ### Note on Support
>
> Whilst this is technically possible now an official server-side compiler hasn't been released. *Official* support for server-side compilation could be added in future releases.
---
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