[question] How can I add a new Theme in V7.0

[diderich]

I would like to customise the current 'Twilight' theme (by adding a new one 'CustomeTwilight') by switching to a dark mode, change the background colour of the header and footer, have the menu background a different colour (grey) than the data, adjusting row heights on the summary pages, etc ...

What is the best/most appropriate way of doing this (especially avoiding to lose the changes when installing a new version/release)?