Thanks Davo,
I actually work with Debian Sid Kde, where I am very happy with lazarus QT applications. (GTK also perfect). Although the application drinks from the system themes, it is possible to change colors from lazarus to controls, that is, perfect customization to create a Dark Theme independent of the distribution.
The problem is the tests I have done for Windows, where the default theme is light with blue touches. (very similar to the native win widgetset). Unable to create a "Dark Theme".
For example in GTK2 for Windows, you can attach a complete theme to a folder inside the executable and thus distribute the application with a custom theme. In QT I do not see this possibility, except that from QT Creator or similar (which I am completely unaware of) .dll's can be generated with custom themes.
Even so, I think that the default theme for win that lazarus compiles for QT5, is much more stylized, since the one with Windows by default has bright and disproportionately wide scrollbars that break any possibility of an application with its own style.