Hi, 

External style sheets has been added in Odoo, in report module, which contains classes of bootstrap, font-awesome etc.

You can refer to  file : report > views > layouts.xml

In which you can find the external stylesheet links:

 <xpath expr="//head" position="inside">

<link href="/web/static/lib/bootstrap/css/bootstrap.css" rel="stylesheet"/>

<link href="/website/static/src/css/website.css" rel="stylesheet"/>

<link href="/web/static/lib/fontawesome/css/font-awesome.css" rel="stylesheet"/>

<style type="text/css">

<t t-call="report.style"/>

</style>

</xpath>

here template with id "report.style" is called, in which some new styles are defined:

in the same file, you can find this template:

<template id="style">

body {

color: #000 !important;

word-wrap: break-word;

}

.border-black td {

border-top: 1px solid black !important;

}

.table-condensed > thead > tr > th {

border-bottom: 1px solid black !important;

}

.table-condensed > tbody > tr {

page-break-inside: avoid !important;

}

.zero_min_height {

min-height: 0px !important;

}

</template>

Likewise, you can explore all other css classes defined in Odoo.

Hope, this helps you !