OpenOffice to LibreOffice Shortcut usage comparison Chart?

OpenOffice to LibreOffice Shortcut usage comparison Chart?

example when entering dates OpenOffice “.” will separate the numbers but in LibreOffice “/” is used.
05.05.21 = May/21/2021

05/05/21/ = May/21/2021

Is there a Dark Theme mode for any of the Libreoffice software?
Request: Dark Theme for all LibreOffice apps.

To avoid the confusion that your example shows, you should always use the format YYYY-MM-DD, so for your example it would be 2021-05-05 which is understandable by anyone from anywhere.

I am not tempted to customise so I won’t try to compare themes but have you tried Tools > Options > LibreOffice > Personalisation?