for this style, you need the pre-December 2012 style. to get that style you need either a cookie:
open a Youtube page,
open your console
Ctrl + Shift + K on Firefox
Ctrl + Shift + J on Chrome
Ctrl + Shift + I on Opera
and paste this javascript:
document.cookie="VISITOR_INFO1_LIVE=qDpUsBNO0FY; expires=Thu, 2 Aug 2020 20:47:11 UTC"

(that might not work, from personal experience, and is more complicated)

or use this userscript (greasemonkey needed):
http://userscripts.org/scripts/show/153968

go on youtube, enable the script, refresh the page, disable the script, refresh the page again, you have pre-december theme, enable this style (you will need to do this each time you close and open the browser...)(enabling te userscript and not disabling it may cause a glitch where multiple instances of the cookie come- if this happens, disable the script, restart browser, enable, go to youtube, disable script, refresh, enable style)
if you use the current theme with this it looks very glitchy


--------------------------------------Versions-----------------------------------

1.0 - 1st public version
1.1 - removed fade effect on items, fixed channel result and title text size, added style of old 'create account' button to 'browse channels button', white text in the black 'active' buttons, added border line at filter, edited HD button to match old one
BUGS: transparent '[insert #] videos' button text
1.2 - bugs fixed, user button text easier to read, more (old-style) borders added
BUGS: buttons not correct size in name change dialogue for reasons not to change (in that dialogue just disable it, select a reason, then re-enable

--------------------------------------Todo--------------------------------------
-Make channel pages look prettier (old is probably much impossible without userscripts)
-old comment #likes/dislikes icons
-Upload large image versions to image website (not chosen yet...)
-put link to userscript for old icon in description
-------------maybe----------------
-create grey-scale classic youtube icon (for footer/bottom of page)
-fix playlist icon hover
-increase button sizes to old sizes...
-old video border
-suggestions that would be able to be done with userstyle
-learn userscripts and make html5player look like player4
-old video response box