You would like to read my post How to install themes in Google Chrome if you don’t know how to install them.
IMPORTANT: Do make the backup of the original theme file before replacing it with these new ones.

1 – Go Green

A mix of green colors in a truly “Green” environment. This is a theme for all of you who care about our planet. Go Green!
Download | Theme Author

2 – No Icons

A stab at a Google Chrome theme without the icons on the bar. I could only fit 4 characters per button so…
Download | Theme Author

3 – Mac V1.0 Theme

The theme has been inspired by Mac Osx
Download | Theme Author

4 – BlurkMAC – Mac OS X Theme

Download | Theme Author (not Available)

5 – BlurkDev DeviantART Theme

Download | Theme Author

6 – Black Window Live

Download | Theme Author

7 – IE7 Google Chrome Theme with Vista/Aero look

Download | Theme Author (Not Available)

8 – Google Chrome Firefox Vista Theme

Download | Theme Author (Not Available)

9 – Google Chrome Milk Theme

This is a theme was made to blend with the Milk 2.0 msstyle for OS X.
Download | Theme Author

10 – Safarish Theme

Anyone likes Safari here ? No ? okk.. anyone likes graphics/colors of Safari here.. For them we present Safarish.. The Safari like Theme for Google Chrome
Download | Theme Author (not available)

Create a new bookmark and call it “Download YouTube” or something you will remember, then for the address field paste:

Quote:

javascript:window.location.href = ‘http://youtube.com/get_video?video_id=’ + swfArgs['video_id']+ “&l=” + swfArgs['l'] + “&sk=” + swfArgs['sk'] + ‘&fmt_map’ + swfArgs['fmt_map'] + ‘&t=’ + swfArgs['t'];

Then save your bookmark. Next go to the YouTube video you want to download, when it starts playing click your “Download YouTube” bookmark and you will see the .flv start to download in Chrome’s download status bar.

Download Youtube Videos in MP4
With a bit of tinkering around with the above code, also downloading the video in MP4 is possible.

Same as above, create a new bookmark (It’s probably best to name the above Download YouTube in FLV and then name this one Download YouTube in MP4) then add the following code in the address field:

Quote:

javascript:window.location.href = ‘http://youtube.com/get_video?video_id=’ + swfArgs['video_id']+ “&fmt=18″ + “&l=” + swfArgs['l'] + “&sk=” + swfArgs['sk'] + ‘&fmt_map’ + swfArgs['fmt_map'] + ‘&t=’ + swfArgs['t'];

Save then use the same way as the above.