Yeah, it's clearly their internal bug cuz it can't find the lightgray skin located at https of chess.com. They made some changes yesterday since it didn't appear before. Btw I use PC, Windows 7, Firefox and v3 of chess.com. 
Failed to load content css
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Aug 4, 2017
0
#2
Aug 4, 2017
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#4
Does anyone work on chess.com to fix internal bugs ?!?
Please put again this f!cking lightgray skin at its location. 
Aug 4, 2017
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#5
Mecanicas wrote:
Esteemed chessmate Igor;
There is no need to use vulgar or offensive words. They will fix it as soon as they can.
Are U sure ?!? It's been long time that I didn't see any technical support here when I asked it.
Aug 4, 2017
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#6
I rarely complain but this warning message appears constantly and disturbs the correct use of forums here.
Aug 6, 2017
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#10
Happened to me for the first time today and I sent them a screenshot.That was before I found this forum. Perhaps it has taken a while to travel this far South. I'm sure they're working on it as a major priority.
Aug 7, 2017
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#11
Karnakatz wrote:
Perhaps it has taken a while to travel this far South.
LMAO 
Aug 7, 2017
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#12
Thanks for the reports. We have identified the problem and applied a patch. You should not see this issue any more, but please do report again if it re-appears for you before we apply the permanent fix.
Aug 7, 2017
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#13
bcurtis wrote:
Thanks for the reports. We have identified the problem and applied a patch. You should not see this issue any more, but please do report again if it re-appears for you before we apply the permanent fix.
All right, thanks a lot! I confirm that now it works well for me. 
I've been getting this same error for the last several days. 
May 20, 2018
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#16
WhiteDrake wrote:
I've been getting this same error for the last several days.
I suggest you PM bcurtis, above, Jan.
May 21, 2018
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#17
@WhiteDrake can you copy the exact error you are getting? The URL above is working fine.
I was getting this exactly same error message:
Failed to load content css: https://betacssjs.chesscomfiles.com/bundles/app/js/vendor/tinymce/skins/v2/lightgray/content.min.css
The error is gone after I've deleted cached web content and chess.com cookies, signed out and signed in again. So some combination of it has probably worked. (Sorry, I didn't save the error messages in the console.)
This warning message constantly appears since yesterday:
Failed to load content css: https://betacssjs.chesscomfiles.com/bundles/app/js/vendor/tinymce/skins/v2/lightgray/content.min.css