A rule, coming from I don't know where, overrides a border-top rule in https://en.wikipedia.org/wiki/Module:Navbox/styles.css, which leaves navboxes such as https://en.wikipedia.org/wiki/Template:States_of_Brazil in a weird state in dark mode, with some inner borders brighter than the others.
Per https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_220#My_question:_WHYYYYYYY%3F%3F%3F, it seems like the source of this rule is in the WikimediaMessages dark mode CSS, not local code.
Rather, all the other dark mode code associated with navboxes is in WikimediaMessages. The rule associated with the white borders is onwiki. As I suggested there and elsewhere previously, it would make sense for this rule to live with the rest of the stuff in WikimediaMessages.
The styles in WikimediaMessages were temporary and generic so they could apply to all wikis to buy wikis time to develop their own style adjustments on wiki and not being actively developed and we are generally not accepting new patches there. The expectation is communities disable them and ship their own following the instructions in https://www.mediawiki.org/wiki/Extension:WikimediaMessages#Disabling_styles if they are not fit for purpose.
If you need something that will help you completely wean your project off those rules let me know and I will help that get prioritized.