How can i achieve the old school fltk title window and window buttons? #1413

There is no such thing like "old school fltk title window and window buttons". The screenshot you present to us is obviously from our (FLTK) screenshots page.

The window decorations are created by a part of the system usually called the "Window Manager" (WM), and what you think is "old school" is in fact a screenshot from a particular old Window manager, likely from a Linux system. On Windows systems the WM is Windows itself, and the entire window decorations are created by the OS (not by FLTK). As you can also see on the screenshots page, there is at least one different screenshot from an older version of macOS:

and there are more (from fluid) of another OS or WM.

Short answer: since the window decorations are created by the OS or WM, you can't change this in FLTK. It's up to the system (version) you are using. There may be some "tweak tools" on your system to change the look of windows, but that's nothing we can answer here.