There are plenty of websites that inspires me. If I like design or their css or javascript, can I copy it to my local folder and use it?
To give you an example, if I find a site xyz.com and liked their login screen. Can I copy their css, background images (plain gradient ones) and create my own version with little modification?
There is plenty of copying going around on the internet. It is difficult to stay away from. But to honestly answer your question it is illegal to copy someone else work, unless you get consent from the owner, but be aware that this is the unique work of the owner and not copied either.
Now a lot of work is not that unique and is pretty much the standard way of developing say javascript, so if thats the case I do not see a problem with it. But related to actually copying images, css, styles, colors, it is tricky. If the techniques are generally used and you create your own images and different colors it can be acceptable, otherwise I suggest coming up with your own. Also stay away from copying from recognized brands.
I do believe we can use other websites to our advantage and get inspired by designs, techniques, solutions and help it to drive our own creativity.
CSS code is not copyrightable by itself insofar as the markup goes.
However, owners can finagle their CSS implementation into something called trade dress, which is fancy-talk for how a company distinguishes itself from other competing companies.
To summarise a very long, long argument: copyright can be applied to the distinguishing design of a website. However, the markup as an entity cannot be by itself copyrighted.
Because HTML / CSS is not machine code (its markup or script), copyright laws can only be loosely applied to it.
Once you bring efficiency and algorithmic programming into the argument, those can be copyrighted in the United States. Therefore, if you, somehow, program a "special" algorithm into CSS, then the owner has full copyright over that block of code. He / she cannot, in any circumstance, copyright something like
a:link {text-decoration: none}