Sorry, native customization of the standard interface is not a feature of Sheets. Despite their size, Google still say they welcome feedback. If you have a problem with a Google product, especially related to accessibility, choose Report A Problem from the Help menu and tell them about it.
Okay, with that said, this is totally doable. Further, because of how websites are delivered to browsers, the editing possible here is possible on almost every site. The coloration and other style settings of elements of webpages are just determined by a plain text list of "CSS" rules (see: tutorial) sent to your browser along with the page.
background-color: #484848 !important;
The the above CSS rule, for example, selects tabs other than the active tab, and forces them to have dark gray backgrounds.
Making a specific recommendation for editing style sheets in your browser is a bit off topic in this site, but if you think this approach is suitable, now at least you know it's an option, and what to search for. I captured the above-pictured alteration after modifying my Firefox profile's userContent.css (see: intro, ancient but first-party article, wiki page, tutorial on even editing the browser GUI), but the same could be achieved through other methods like browser add-ons (Firefox, Chrome, Safari, Opera).