Is it possible to alter a CSS stylesheet using JavaScript? (NOT the style of an object, but the stylesheet itself)

As of 2011

Yes you can, but you will be facing cross-browser compatibility issues:

http://www.quirksmode.org/dom/changess.html

As of 2016

Browser support has improved a lot (every browser is supported, including IE9+).

• The insertRule() method allows dynamic addition of rules to a stylesheet.

• With deleteRule(), you can remove existing rules from a stylesheet.

• Rules within a stylesheet can be accessed via the cssRules attributes of a stylesheet.

We can use a combination of .insertRule and .cssRules to be able to do this all the way back to IE9:

function changeStylesheetRule(stylesheet, selector, property, value) {
    // Make the strings lowercase
    selector = selector.toLowerCase();
    property = property.toLowerCase();
    value = value.toLowerCase();
    
    // Change it if it exists
    for(var i = 0; i < stylesheet.cssRules.length; i++) {
        var rule = stylesheet.cssRules[i];
        if(rule.selectorText === selector) {
            rule.style[property] = value;
            return;
        }
    }
  
    // Add it if it does not
    stylesheet.insertRule(selector + " { " + property + ": " + value + "; }", 0);
}

// Used like so:
changeStylesheetRule(s, "body", "color", "rebeccapurple");

Demo

When I want to programmatically add a bunch of styles to an object, I find it easier to programmatically add a class to the object (such class has styles asscociated with it in your CSS). You can control the precedence order in your CSS so the new styles from the new class can override things you had previously. This is generally much easier than modifying a stylesheet directly and works perfectly cross-browser.

change a property in a style rule

function change_css_style (titulo,selector,propiedad,valor) {        
        let i=0;
        while (i<document.styleSheets.length) {
            if (document.styleSheets[i].title==titulo) {
                let y=0;
                while (y<document.styleSheets[i].cssRules.length) {
                    if (document.styleSheets[i].cssRules[y].selectorText==selector) {                                               
                        document.styleSheets[i].cssRules[y].style[propiedad] = valor;                                                                       
                        y = document.styleSheets[i].cssRules.length;
                    } 
                    y++;
                }               
                i=document.styleSheets.length;
            } 
            i++;
        }

    }

DEMO

<style title="chat_inicio">
    .contenido .mensajes {
          width: 100px;
          height: 300px;    
    }
</style>

change the style book with the title chat_inicio with the selector .contenido .mensajes the property of the style width to 475px

<script>
     cambiar_css_style ('chat_inicio','.contenido .mensajes','width','475px');
</script>

2020

Some advantages of this method:

• Does not require (but allows) stylesheet to be specified.
• Allows multiple styles to be added / modified at once
• Accepts !important attribute
• Ignores extra whitespace when matching CSS selector
• Changes last matching existing rule, or appends to end of last matching stylesheet. (Other answers add/change the first rule which may be then overruled.)

Usage:

adjustCSSRules('#myDiv', 'width: 300px !important');

Method:

function adjustCSSRules(selector, props, sheets){

    // get stylesheet(s)
    if (!sheets) sheets = [...document.styleSheets];
    else if (sheets.sup){    // sheets is a string
        let absoluteURL = new URL(sheets, document.baseURI).href;
        sheets = [...document.styleSheets].filter(i => i.href == absoluteURL);
        }
    else sheets = [sheets];  // sheets is a stylesheet

    // CSS (& HTML) reduce spaces in selector to one.
    selector = selector.replace(/\s+/g, ' ');
    const findRule = s => [...s.cssRules].reverse().find(i => i.selectorText == selector)
    let rule = sheets.map(findRule).filter(i=>i).pop()

    const propsArr = props.sup
        ? props.split(/\s*;\s*/).map(i => i.split(/\s*:\s*/)) // from string
        : Object.entries(props);                              // from Object

    if (rule) for (let [prop, val] of propsArr){
        // rule.style[prop] = val; is against the spec, and does not support !important.
        rule.style.setProperty(prop, ...val.split(/ *!(?=important)/));
        }
    else {
        sheet = sheets.pop();
        if (!props.sup) props = propsArr.reduce((str, [k, v]) => `${str}; ${k}: ${v}`, '');
        sheet.insertRule(`${selector} { ${props} }`, sheet.cssRules.length);
        }
    }

Demo

The method takes three arguments:

• selector [String] - CSS selector - eg: '#myDiv'
  Whitespaces are auto-reduced (.myClass #myDiv will match .myClass #myDiv)
• rules [CSS String, Object] - eg (either is acceptable):
  • { border: "solid 3px green", color: "white" }
  • 'border: solid 3px green; color: white'
• sheet (Optional) [String, StyleSheet]
  • if empty, all stylesheets will be checked
  • 'myStyles.css' A relative or absolute URL to sheet
  • document.styleSheets[1] - A reference to a sheet

Other examples:

adjustCSSRules('#myDiv', {width: '30px'}); // all stylesheets
adjustCSSRules('#myDiv', 'width: 30px', 'style.css'); // style.css only  
adjustCSSRules('#myDiv  .myClass', 'width: 30px', document.styleSheets[0]); // only first stylesheet