The Crystal Reports Viewer turns everything into a control that exists on the page. To make it look the way they want, you need to add a virtual directory that you can link to if you right-click and “browse the source” on your web page.
When you do this, you'll see a helper directory referenced countless times to access all of the JavaScript files for images and CSS. (aspnet_client \ system_web \ 2_0_50727 \ crystalreportviewers13 - ONE example of the path that you can see, depending on the version of Crystal Reports you are using).
My point is that the CSS link should fall under this path. The problem created on your site is probably related to the problem that arises due to how CSS is applied to the page. Unfortunately, CSS, like JavaScript objects, can have conflicts with other elements on the page. They are not "self-sufficient" in nature.
A BIG way to see the exact location of CSS related issues is to install the Web Developer add-on for FireFox or Internet Explorer. With this setup, you can go to the "CSS" menu and see a list of CSS files that affect the site. In fact, you can even turn off CSS by location and see how it affects the LIVE page. You can also click on the objects on the page and it will reveal the CSS details that apply to the objects. Alternatively or also, you can install the FireBug add-on. It got a great interface and can provide you with other tools to learn it.
Good luck isolating CSS. After you find it, you need to either customize your existing CSS to use it! Important tags to override their styles or some combination of markup and CSS settings. I do not recommend customizing the Crystal Report CSS viewer; it is safer and more convenient to maintain YOUR CSS to edit in order to avoid a problem.