The Breeze dark theme has very little color differencing between the title bar and the application toolbars (the ones that come right beneath the title bar). The light breeze theme has a more clear separation (light gray vs. darker gray) and looks better that way. Would be more appropriate to use a darker color for the active title bar for breeze dark too (#1e1e1e would be a good choice).
This should have been fixed already in Plasma 5.21; the whole header area is a unified color, and changes color for active vs inactive windows. What version of Plasma are you using? Can you attach a screenshot showing the active and inactive states?
Created attachment 141191 [details] active_and_inactive_current Here is a screenshot using a recent OpenSuSe Tumbleweed with the latest plasma (a different setup from where I was experimenting). The breeze dark theme merges the title bar with what's below, while the regular breeze theme doesn't (it sets a darker gray for the title bar). In order to have a colored titlebar with dark breeze, one would have to switch to a different color scheme such as "Breeze high contrast".
Yes, this is the intended appearance. Believe it nor not, before we unified the titlebar and toolbar colors, there was *zero* difference between the active and inactive window titlebar color in Breeze dark. If you'd like *even more* contrast between active and inactive windows, you're welcome to change the Header Normal Background/Alternative Background colors to be less similar.