I assume you're looking for this: StylesSheetManager - A 'CSS like component' for WinForms
Try to post your question in the comment section under the article, so that the author of the component gets at least notified of your query. Though it seems that he's not supporting it anymore.
Your best bet will be to read the article. I guess the questions you (may) have, are already answered there.
Anyways, to add a component to the VS Toolbox, right-click the toolbox, select the
option, click browse, navigate to the library containing the control, it should get added to the list, check the checkbox in the list, click
. Easy as that.