Update index.scss

[ttencate]

Hi, thanks for your contribution! Could you explain what this is for and which problems it solves? Please update the commit message so we have a clear history of why these changes were made.

[Nep-Toodles]

This is for improving design and making it look mordern

[ttencate]

I literally don't see any difference:

• The background-color on the html element is immediatelly overridden by background-image (which is a gradient so there's no loading time either, during which the background-color could be visible).
• The background and color on input elements are overridden by the input[type="text"] rule directly below and .floatslider further on. I don't think we have other types of input elements.
• Similarly, those on button elements are overridden by the .button class.

Where in the UI does this actually make a visible difference?