This bug does not exist in v126.96.36.199 (x64): creating "New Theme" works fine.
In Draw v188.8.131.52 (x64), when I click on "New Theme" button, a window quickly pops up and quickly (~0.5s) disappears (window is not hidden behind the main application window. it is completely gone) and leaves Draw open, but in a locked / frozen state. The mouse no longer works in application window, but I can still move main window around. Clicking on "X" to close main window does nothing. I have to End Task in Task Manager.
It should be fixed in 5.3.1.
Don't hesitate to reopen this tracker if you still reproduce this with 5.3.1 (expected at the end of February or beginning of March, see https://wiki.documentfoundation.org/ReleasePlan)
*** This bug has been marked as a duplicate of bug 105017 ***