I've updated my Google Pixel to Oreo. Now the statusbar and quick settings area have a light theme. Is it possible to change that back to the dark theme, like on previous versions of Android? The white background is way too bright... I didn't find any option in the settings.
Sony helped Google in implementing native support of RRO layers in Android O. This was previously used in many Xperia phones for a better system-wide theme support. In android custom rom and root community, Project Substratum is used for same. It also uses RRO layers thus provides better and more consistent theme than previous option CynogenMod Theme Engine (CMTE).
Till Oreo RRO layer patches and root were necessary for Substratum to work but as Oreo has native RRO layer support so it can be achieved in stock rom.For detailed method see this and this xda article. @Tony Chiboucas has also mentioned the method in his comment.
P.S. For unrooted devices, this is a paid process.