Assuming you have inserted your HTML in an article, what you can do is install Sourcerer which allows you to add custom code to your articles. Once installed you can use the following to import a CSS file:
{source}
<?php
JHtml::_('stylesheet', JUri::root() . 'path/to/your/file.css');
?>
{/source}
or if you don't want to import a file, you can just insert the CSS like so:
{source}
<style>
#element {
/* some code */
}
</style>
{/source}
If Sourcerer seem like quite a heavy plugin to use, you can also download DirectPHP else you could developer a small module
.cssfile and refer to it from your html pages(<link href="~/Styles/Site.css" rel="stylesheet" type="text/css" />). Not sure if Joomla needs anything specific added for that though.