Will offer my way of fixing.. probably something went wrong in the themes, aside from css, probably version mismatches with the core nodebb version your using. The below will revert to working vanilla and lavender theme versions that match w/ your core nodebb install version. (probably need to stick with whatever theme versions it reinstalls, because I've always had problems w/ updating themes, I don't upgrade them anymore) Stick with what came with the nodebb install originally. This will revert back to originals:
@jiangcaiyang It works if you do it right. It doesn't take LESS variables, but it does work. Odds are the less styles have higher specificity, so your custom CSS rules are not applied. Use !important or something instead. It will certainly benefit you to not be held back due to incompatibility with newer versions of NodeBB in the future.