Matching your theme
Because every colour and measurement is editable (see Colours and Shape and type), the bar can be made to look like it was built into your theme, rather than dropped on top of it. This page is a starting approach, not a rule.
Start from your theme's own values
Most themes use a consistent set of colours for buttons, badges or tags elsewhere on the site. Matching those, your theme's button background for the selected state and its border colour for resting tags, usually reads as more "native" than picking a new palette from scratch.
Use the preview honestly
The Design page's preview includes a page-background switcher (Light, Warm, Dark) so you can check the bar against something closer to your theme's actual background than the app's own admin surface. If your storefront uses an off-white or dark background, checking contrast there, not just against pure white, catches problems before they reach real readers.
Radius and spacing carry more of the "match" than colour
Two bars in your exact brand colours can still look mismatched if one uses sharp corners and tight spacing while your theme's buttons are soft and generous, or the reverse. If nothing else, matching corner radius (see Shape and type) to your theme's buttons is often the single change that makes the biggest difference.
Tip
There's no requirement to match your theme exactly. A bar that intentionally stands out from the surrounding page is a valid design choice too. This page describes how to blend in, for stores that want that.