**Disclaimer - I work in tech consulting but do not profess to be a design\dev\UX expert**
I've been using the site for years, and it did need updating. It was dated for mobile use, and not the quickest to work with, with a hit/miss search facility. Particularly with the parent company running a different ecommerce platform, it made sense to update. I'm all for change.
Visually what has happened though, is that the OCUK/CK web team (or 3rd party contractor) very hastily re-skinned the caseking.de theme to bring platform parity, and didn't really test this through thoroughly enough or announce with customers to help with the migration process. As a result, the colours in the theme are a bit too strong (as many have said) and a few areas haven't been rebranded ('Rainforest' sign-in info still auf deutsch, some titles/meta still Caseking branded etc). It not deal breaking, but it is frustrating and as a result does make learning the new browsing experience less pleasant than it should, and will inevitably irk the anti-change brigade more than it reasonably should.
The major sin here for me is not using the opportunity to upgrade both OCUK and caseking to a better theme from the perspective of customer experience. I'm sure there are valid backend/inventory/fulfillment reasons as to why this particular platform was chosen, but the lack responsive design to cater for mobile/tablet friendly display formats (and for ultrawide/high resolution browsing - I imagine a good number of customers view the site at above 1080p?) and the missed opportunity to streamline/improve the shopping experience seems negligent and poorly thought through.
The Sunday launch seems odd, as there were no OCUK support staff working to answer queries and lack of info for forum members about free shipping, accounts moving etc was surprising. This didn't feel like the OCUK we know and love.
Themes change, technology changes. A bit of post launch triage will fix the apparent problems. It's the service, people and ethos that keeps me personally as a customer anyway. Don't expect that will have changed.