Allons enfant de la Patrie...
Les coulours de la drapeau de la France (bleu foncé - Colours of flag of France (dark blue)

In Plasma 5.24 install Lightly from https://github.com/Luwx/Lightly.
In Plasma 5.25 in order to have blurred Titlebar install Lightly from https://github.com/boehs/Lightly.
Choose Transparent Dolphin view in Appearance/Application Style/Lightly/Settings/Frames tab and adjust Sidebar transparency in Transparency tab.
Remember also to choose Lightly in Window Decorations!
A very good Lightly installation manual in YouTube:https://www.youtube.com/watch?v=VIrYPFY25G4