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I am using the theme tag not the widget, can I restyle it?

At the moment there is no interface for that.

There is a workaround however.

1. Drag the widget into any sidebar (even the tabbed sidebar itself)
2. Use the widget's settings to style the CSS
3. Save the widget settings
4. Remove the widget from the sidebar
5. Resave the settings

I't ain't elegent, but it works.

The default styles are not working properly, what's going on?

CSS is tricky to write for all situations so it is possible that it is being overidden by other, more specific settings. If possible do not include the sidebar in multiple nested lists, that can cause problems. To diagnose the problems try moving the sidebar so it is not inside any element except a div tag. If that works the problem is that the CSS where you want to place the sidebar is overriding the default.