markdown-viewer/js/content.js

function injectCSS () {
    var link = document.createElement('link');
    link.rel = 'stylesheet';
    link.type = 'text/css';
    link.href = '#';
    link.id = 'theme';
    document.head.appendChild(link);
};

$(function () {
    injectCSS();
    
    $('body').addClass('markdown-body');//github
    $('pre').attr('id', 'markdown').hide();

    chrome.extension.sendMessage({
        message: 'markdown',
        markdown: $('#markdown').text()
    }, function (res) {
        $('body').append('<div id="html">').find('#html').append(res.marked);
        Prism.highlightAll();
    });

    chrome.extension.sendMessage({
        message: 'settings',
    }, function (data) {
        $('#theme').attr('href', chrome.extension.getURL('/themes/'+data.theme+'.css'));
        
        $('#theme').attr('disabled', data.raw);
        $('#markdown')[data.raw?'show':'hide']();
        $('#html')[data.raw?'hide':'show']();
    });
});

chrome.extension.onMessage.addListener(function (req, sender, sendResponse) {
    switch (req.message) {
        case 'reload':
            window.location.reload(true);
            break;
        
        case 'theme':
            var raw = $('#theme').attr('disabled') == 'disabled';
            $('#theme').remove();
            injectCSS();
            $('#theme').attr('href', chrome.extension.getURL('/themes/'+req.theme+'.css'));
            $('#theme').attr('disabled', raw);
            break;

        case 'raw':
            $('#theme').attr('disabled', !($('#theme').attr('disabled') == 'disabled'));
            $('#markdown, #html').toggle();
            break;
    }
});