Svelte has an open issue because of a "feature" that adds duplicate classses which breaks the expected cascade in favor of some inexplicable specificity. Instead of having to go through all your override/modifier rules and add !important
to them, you can just add this WP plugin to strip out the duplicate classes.
// in ./.webpack/loader.remove-duplicate-svelte-classes.js
module.exports = function removeDuplicateSvelteClasses(source) {
const SVELTE_RULE_REGEX = /\.svelte-[a-z0-9]+/g;
const ruleMatches = (source.match(SVELTE_RULE_REGEX) || []);