TRAC - change css but being overwrote on page load

Does anyone know how I can change a css files like roadmap.css for trac? I figured out where the file is but when I modify it, the modification is overwrote on page load/refresh.

Example of where my roadmap css is – x:\xx\xx\BitNami Trac Stack projects\.egg-cache\Trac-0.12.1-py2.5.egg-tmp\t开发者_开发百科rac\htdocs\css\roadmap.css

I would like to add some different classes for the milestone graph like

table.progress td.promote { background: #dcefdc}

Instead of modifying the existing stylesheets, add your custom styles to a new stylesheet and link it into your pages. Your custom styles should override the built-in styles. See TracInterfaceCustomization for more info and CookBook/SiteStyleCss for examples.