The plugin comes with a predefined template, which can be modified in the settings page.
Four themes are included with the plugin, a light version and a dark version, and flat versions of each of those.
If you prefer to roll your own, just add your own CSS, the plugin even has an option to disable built-in styles altogether, so you can start with a blank canvas.
Some details, such as typography and link colour will be dependent on your theme.
To experiment with different styling, use the CodePen widget, or visit the CodePen page below.
See the Pen Onebox Plugin Style Playground by Surreal Road (@surrealroad) on CodePen.