The defaultapp.css

The themes/defaultapp.css file contains some global CSS rules and classes commonly used in web applications.

Box sizing set to border-box

The following rule is applied to facilitate CSS layout.

* {
    box-sizing: border-box;
}

Margin, padding and height of <html> and <body> tags

Margin and padding are set to 0 to avoid user-agent styling. For example, most of browsers set margins on <body>. To facilitate layout of pages that fill the whole available height, height: 100% is set on <html> and <body> tags.

html, body {
    height: 100%;
    margin: 0;
    padding: 0;
}

The width100 and height100 utility classes

These classes are just shortcuts for setting width/height to 100%.

class="width100 height100" 

is equivalent to

style="width: 100%; height: 100%"