I have CentOs and Mageia with KDE 4. I want to configure window decoration. I enter "system settings->workspace appearance->window decoration" and click "get new decorations" button. Empty "System settings add-ons installer" window appears with caption "Initializing" in the bottom. And nothing loads ever! The window stay empty!
May be I need to configure decorations storage url somewhere?

I prepared screenshot but could not provide it here. (I get error: "It was not possible to determine the dimensions of the image.") Sorry.
https://postimg.org/image/4xpymn1rh/

What specific version of KDE?
Most of the theme-related stuff moved recently from opendesktop.org (kde-look.org) to store.kde.org (looks like kde-look.org now redirects to the new store).
You may need to load the new providers.xml file for the new store.
Open System Settings -> Social Desktop. There should be a combo-box with the provider(s) to manage, beside which will be buttons to add and remove providers. If the combo-box doesn't have store.kde.org then add it, using the URL: https://store.kde.org/ocs/providers.xml (and then remove any provider for opendesktop.org).

Thank you very much! I added aforesaid provider to "Social Desktop" and now window decorations become available!

airdrik wrote:What specific version of KDE?
Most of the theme-related stuff moved recently from opendesktop.org (kde-look.org) to store.kde.org (looks like kde-look.org now redirects to the new store).
You may need to load the new providers.xml file for the new store.
Open System Settings -> Social Desktop. There should be a combo-box with the provider(s) to manage, beside which will be buttons to add and remove providers. If the combo-box doesn't have store.kde.org then add it, using the URL: https://store.kde.org/ocs/providers.xml (and then remove any provider for opendesktop.org).

Any idea what to do if one does not have System Settings > Social Desktop?

That was my bad. The Social Desktop module is under System Settings -> Account Details -> Social Desktop.
Though, if you are using Plasma 5, I'm not sure where else it may have moved (I couldn't find it the other night on my PCLinuxOS + Plasma 5 setup)

Thanks airdrik. I'm in OpenMandriva Lx 3/Plasma 5.9.5 and it isn't in our System Settings. Which maybe means it isn't in Plasma 5.9.5.