If you mean that the general rules from the first included styesheet don't apply: That's due to your use of min-device-width
in the first line. Erase that , like:
<link rel="stylesheet" type="text/css" media="screen and (max-device-width: 1500px)" href="style.css" />
<link rel="stylesheet" type="text/css" media="screen and (max-device-width: 480px)" href="mobile.css" />
That way the first stylesheet will be loaded, afterwards the rules from the second stylesheet will overwrite those with identical selectors from the first stylesheet, as you probably intended.
In your version only the second stylesheet will apply to smaller devices, without the first one...
Also, think about if you really want to use max-device-width, or rather just max-width
. A media-query with max-device-width: 480px
will in most cases only apply to devices which are smaller that 240px "CSS pixels" (which hardly exist anymore) due to the double (or more) pixel density of most modern devices.