Because it's a "sample" you could basically throw any css in there.
You're on the right track using inspector tools. If you need further help, you should post a thread in the theme's dedicated support forum on wordpress.org, if you are using one of the free themes provided there.
Duster theme support is here:
Keep in mind that the Duster theme is an old theme (it hasn't been updated in over 2 years), so things may eventually break.
If you are using a commercial theme, you should contact your vendor for support, as you paid for support when you bought the theme.