You can browse the Themes to try to find one you like better. But yeah it'd be nice if one of the default built-in themes was super plain. Closest they have available by default is a grayscale gradient ("Ash").

Here you go: add this to your userChrome.css file.

/disable gradient

/ 1. Remove the toolbar/tab bar gradient completely /

:root:not([theme-image-in-toolbox]) body,

:root[theme-image-in-toolbox] #navigator-toolbox {

    background-image: none !important;

}

/ 2. Strip Nova purple accent color/focus outlines from the URL & search bar */

:root {

    --toolbar-field-focus-background-color: var(--toolbar-field-background-color) !important;

}

.urlbar-input-container, #searchbar {

    --toolbar-field-border-color-focus: var(--toolbar-field-border-color) !important;

    box-shadow: none !important;

}

The gradient is part of the theme. Updated the post with how to roll back the UI changes for now. After rolling back, select an old theme.

I just turned off auto-updates after reading this. Safe for now.