I am using paragraphs jQuery UI Accordion and finding it is impossible to override CSS from jquery.ui.theme.css (a core file) in my theme as the jquery css is being applied after the CSS for my theme.
Particular selectors are .ui-state-default and .ui-state-active
Usually the themes have the last say... I wonder why not in your case.
How did you include your css file? Via entry in the theme's info file or via backdrop_add_css or the like?
Thanks @indigoxela - it was the answer on zulip (https://backdrop.zulipchat.com/#narrow/stream/218635-Backdrop/topic/Over...) that helped in the end, the theme wasn't producing specific enough selectors for these elements.