Apply custom CSS to the text drawn within a dygraph. See the CSS documentation on the dygraphs website for additional details on which styles are available.
Dygraph to add CSS styles to
Path to css file to be used for styling textual elements of the graph.
dygraph with additional CSS styles
See the CSS Styling article on the dygraphs for R website for additional details.
Note that CSS styles are global so will affect all dygraphs on a given web page. This also implies that for a page with multiple plots you only need to specify styles for the first one (alternatively you can just add them directly to the page by other means).
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.