In order to get the closest look within the sidebar, you can use the internal (not well documented) css variables. Here's how I using it. With these settings the colors from the current color theme are used.
Background color: var(--joplin-background-color2)
Hover background color: var(--joplin-background-hover)
Foreground color: var(--joplin-color2)
Divider color: var(--joplin-background-color2)