Ubuntu 21.10 approved the discontinuation of a theme that combines dark titles, light backgrounds, and light controls. Users will be offered a default all light Yaru theme, and will be given the option to switch to a completely dark option (dark headings, dark backgrounds, and dark controls).
The decision is explained by the absence in GTK3 and GTK4 of the ability to define different background and text colors for the title and main window, which does not guarantee the correct operation of all GTK applications when using combined themes (for example, in gnome disk analyzer a white input bar appears in a dark title bar). Another reason is too much effort to maintain non-standard themes. The problem is that GNOME does not provide an official programming interface and set of recommendations for GTK skins, which breaks compatibility with third-party themes in every new release of GNOME.
Other changes expected in Ubuntu 21.10 include a move away from using the eggplant color for the background of radio buttons and widgets (a replacement color has yet to be confirmed).
Source: opennet.ru