You can style most of the form using the settings below. If you want any more just let me know.
To change the style go to Settings > Interest Calculator > Styles
If you are using the registration form the styles are set on the registration form tab.
The form width defaults to 100% (the width of the container) but you can define a max width if you want.
The form border has 5 options and you can set the thickness and the colour.
You also have the option to set a background colour or image
The slider output is the current value of the slider. Normally appears in the centre.
The max and min values display at the left and right ends on the slider
The slider bar the horizontal component of the slider.
The slider handle is the bit you move