@electryon You could use color-mix to add white into the accent color like this:
/* Lighten accent color from page */
#browser.color-accent-from-page {
--colorAccentBgLighter: color-mix(in hsl, var(--colorAccentBg) 50%, white);
}
/* For "Accent on Window" theme setting ON */
#browser.color-behind-tabs-on.color-accent-from-page {
--colorTabBar: var(--colorAccentBgLighter) !important;
}
/* For "Accent on Window" theme setting OFF */
#browser.color-behind-tabs-off.color-accent-from-page > * {
--colorAccentBg: var(--colorAccentBgLighter) !important;
}
You can adjust the 50% in the CSS to change how much lightening takes place.
Screenshot showing new lighter Accent color on the tab bar for YouTube.