You can find several tutorials on our documentation server to create a custom theme. These can be found here. It's not possible to make any adjustments to the tundra or base.css, so we always make a separate custom.css file to overwrite the default styles.
What kind of cross browser problems are you facing?
You could also download a custom theme from the App Store and look at the style sheets how there built.
As Erik said: you can always use your own custom.css to overwrite the base tundra.css. Maybe you could explain what differences you've noticed in which browsers.