I am wondering if there is a way to use custom css for some specific text on my confluence page (not using embedded HTML).
Sorry this is an old question, but for the sake of people who search for an answer to this question: you can use span or div macros and use the custom css to apply whatever style you want to their contents.
If necessary, you could create custom div and span classes to allow for multiple styles to be applied to selections of text.
EDIT: Here is an example of the wikimarkup you could use to do this
{div:class=customCss|style=float:left; margin-right:50px}
Custom text in a div
{div}
So you can either use the div class and apply a style in the custom css for the confluence space, or you can use an inline style for the div.