一个可以帮助您完成任务的有趣项目是JSS。

JSS是一个CSS的创作工具，它允许你使用JavaScript以一种声明性的、无冲突的和可重用的方式描述样式。它可以在浏览器、服务器端或在构建时在Node中编译。

JSS库允许您使用.attach()函数在DOM/head部分中进行注入。

Repl在线版本评估。

更多关于JSS的信息。

一个例子:

// Use plugins.
jss.use(camelCase())

// Create your style.
const style = {
  myButton: {
    color: 'green'
  }
}

// Compile styles, apply plugins.
const sheet = jss.createStyleSheet(style)

// If you want to render on the client, insert it into DOM.
sheet.attach()

为了搜索者的利益;如果你正在使用jQuery，你可以做以下事情:

var currentOverride = $('#customoverridestyles');

if (currentOverride) {
 currentOverride.remove();
}

$('body').append("<style id=\"customoverridestyles\">body{background-color:pink;}</style>");

显然，你可以改变内部css为任何你想要的。

有些人更喜欢纯JavaScript，但它可以工作，并且在动态编写/覆盖样式方面非常健壮。

以下是Vishwanath的解决方案，稍微写了一些注释:

function setStyle(cssRules, aSelector, aStyle){
    for(var i = 0; i < cssRules.length; i++) {
        if(cssRules[i].selectorText && cssRules[i].selectorText.toLowerCase() == aSelector.toLowerCase()) {
            cssRules[i].style.cssText = aStyle;
            return true;
        }
    }
    return false;
}

function createCSSSelector(selector, style) {
    var doc = document;
    var allSS = doc.styleSheets;
    if(!allSS) return;

    var headElts = doc.getElementsByTagName("head");
    if(!headElts.length) return;

    var styleSheet, media, iSS = allSS.length; // scope is global in a function
    /* 1. search for media == "screen" */
    while(iSS){ --iSS;
        if(allSS[iSS].disabled) continue; /* dont take into account the disabled stylesheets */
        media = allSS[iSS].media;
        if(typeof media == "object")
            media = media.mediaText;
        if(media == "" || media=='all' || media.indexOf("screen") != -1){
            styleSheet = allSS[iSS];
            iSS = -1;   // indication that media=="screen" was found (if not, then iSS==0)
            break;
        }
    }

    /* 2. if not found, create one */
    if(iSS != -1) {
        var styleSheetElement = doc.createElement("style");
        styleSheetElement.type = "text/css";
        headElts[0].appendChild(styleSheetElement);
        styleSheet = doc.styleSheets[allSS.length]; /* take the new stylesheet to add the selector and the style */
    }

    /* 3. add the selector and style */
    switch (typeof styleSheet.media) {
    case "string":
        if(!setStyle(styleSheet.rules, selector, style));
            styleSheet.addRule(selector, style);
        break;
    case "object":
        if(!setStyle(styleSheet.cssRules, selector, style));
            styleSheet.insertRule(selector + "{" + style + "}", styleSheet.cssRules.length);
        break;
    }

找到了一个更好的解决方案，它适用于所有浏览器。 使用文档。样式表来添加或替换规则。公认的答案是简短和方便的，但这适用于IE8和更少。

function createCSSSelector (selector, style) {
  if (!document.styleSheets) return;
  if (document.getElementsByTagName('head').length == 0) return;

  var styleSheet,mediaType;

  if (document.styleSheets.length > 0) {
    for (var i = 0, l = document.styleSheets.length; i < l; i++) {
      if (document.styleSheets[i].disabled) 
        continue;
      var media = document.styleSheets[i].media;
      mediaType = typeof media;

      if (mediaType === 'string') {
        if (media === '' || (media.indexOf('screen') !== -1)) {
          styleSheet = document.styleSheets[i];
        }
      }
      else if (mediaType=='object') {
        if (media.mediaText === '' || (media.mediaText.indexOf('screen') !== -1)) {
          styleSheet = document.styleSheets[i];
        }
      }

      if (typeof styleSheet !== 'undefined') 
        break;
    }
  }

  if (typeof styleSheet === 'undefined') {
    var styleSheetElement = document.createElement('style');
    styleSheetElement.type = 'text/css';
    document.getElementsByTagName('head')[0].appendChild(styleSheetElement);

    for (i = 0; i < document.styleSheets.length; i++) {
      if (document.styleSheets[i].disabled) {
        continue;
      }
      styleSheet = document.styleSheets[i];
    }

    mediaType = typeof styleSheet.media;
  }

  if (mediaType === 'string') {
    for (var i = 0, l = styleSheet.rules.length; i < l; i++) {
      if(styleSheet.rules[i].selectorText && styleSheet.rules[i].selectorText.toLowerCase()==selector.toLowerCase()) {
        styleSheet.rules[i].style.cssText = style;
        return;
      }
    }
    styleSheet.addRule(selector,style);
  }
  else if (mediaType === 'object') {
    var styleSheetLength = (styleSheet.cssRules) ? styleSheet.cssRules.length : 0;
    for (var i = 0; i < styleSheetLength; i++) {
      if (styleSheet.cssRules[i].selectorText && styleSheet.cssRules[i].selectorText.toLowerCase() == selector.toLowerCase()) {
        styleSheet.cssRules[i].style.cssText = style;
        return;
      }
    }
    styleSheet.insertRule(selector + '{' + style + '}', styleSheetLength);
  }
}

函数的用法如下。

createCSSSelector('.mycssclass', 'display:none');

IE 9。现在可以加载文本文件并设置样式。innerHTML属性。所以本质上你现在可以通过ajax加载一个css文件(并获得回调)，然后像这样设置样式标签内的文本。

这适用于其他浏览器，不确定多久以前。但只要你不需要支持IE8，它就可以工作。

// RESULT: doesn't work in IE8 and below. Works in IE9 and other browsers.
$(document).ready(function() {
    // we want to load the css as a text file and append it with a style.
    $.ajax({
        url:'myCss.css',
        success: function(result) {
            var s = document.createElement('style');
            s.setAttribute('type', 'text/css');
            s.innerHTML = result;
            document.getElementsByTagName("head")[0].appendChild(s);
        },
        fail: function() {
            alert('fail');
        }
    })
});

然后你可以让它取出一个外部文件，比如mycss。css

.myClass { background:#F00; }

function createCSSClass(selector, style, hoverstyle) 
{
    if (!document.styleSheets) 
    {
        return;
    }

    if (document.getElementsByTagName("head").length == 0) 
    {

        return;
    }
    var stylesheet;
    var mediaType;
    if (document.styleSheets.length > 0) 
    {
        for (i = 0; i < document.styleSheets.length; i++) 
        {
            if (document.styleSheets[i].disabled) 
            {
                continue;
            }
            var media = document.styleSheets[i].media;
            mediaType = typeof media;

            if (mediaType == "string") 
            {
                if (media == "" || (media.indexOf("screen") != -1)) 
                {
                    styleSheet = document.styleSheets[i];
                }
            } 
            else if (mediaType == "object") 
            {
                if (media.mediaText == "" || (media.mediaText.indexOf("screen") != -1)) 
                {
                    styleSheet = document.styleSheets[i];
                }
            }

            if (typeof styleSheet != "undefined") 
            {
                break;
            }
        }
    }

    if (typeof styleSheet == "undefined") {
        var styleSheetElement = document.createElement("style");
        styleSheetElement.type = "text/css";
        document.getElementsByTagName("head")[0].appendChild(styleSheetElement);
        for (i = 0; i < document.styleSheets.length; i++) {
            if (document.styleSheets[i].disabled) {
                continue;
            }
            styleSheet = document.styleSheets[i];
        }

        var media = styleSheet.media;
        mediaType = typeof media;
    }

    if (mediaType == "string") {
        for (i = 0; i < styleSheet.rules.length; i++) 
        {
            if (styleSheet.rules[i].selectorText.toLowerCase() == selector.toLowerCase()) 
            {
                styleSheet.rules[i].style.cssText = style;
                return;
            }
        }

        styleSheet.addRule(selector, style);
    }
    else if (mediaType == "object") 
    {
        for (i = 0; i < styleSheet.cssRules.length; i++) 
        {
            if (styleSheet.cssRules[i].selectorText.toLowerCase() == selector.toLowerCase()) 
            {
                styleSheet.cssRules[i].style.cssText = style;
                return;
            }
        }

        if (hoverstyle != null) 
        {
            styleSheet.insertRule(selector + "{" + style + "}", 0);
            styleSheet.insertRule(selector + ":hover{" + hoverstyle + "}", 1);
        }
        else 
        {
            styleSheet.insertRule(selector + "{" + style + "}", 0);
        }
    }
}





createCSSClass(".modalPopup  .header",
                                 " background-color: " + lightest + ";" +
                                  "height: 10%;" +
                                  "color: White;" +
                                  "line-height: 30px;" +
                                  "text-align: center;" +
                                  " width: 100%;" +
                                  "font-weight: bold; ", null);