To solve this problem, you should implement the use of MEDIA QUERY...
<pre>
//CSS Part
body {
background-color: lightgreen;
}
@media screen and (max-width: 300px) {
body {
background-color: lightblue;
}
}
</pre>
In the BODY section
<pre>
//Add your content such as images or paragraphs...
</pre>
For a demonstration (How to use visit): http://www.w3schools.com/cssref/tryit.asp?filename=trycss3_media_example1
By resizing your browser, you will see the background color change according to the media query specified.
Another option is to use Container-fluid, which adjusts its width based on the device's screen size and sets all contents accordingly.