What are the best Figma tools/plugins for managing themes?

The best Figma plugin for managing multiple themes is Tokens Studio plugin.

If you are using Material design, check out Material Theme Builder.

The Material Theme builder is built to assist in exploring the possibilities of dynamic color, harmonizing brand colors, and customizing a type scale. All with implementation in mind to bridge designer and developer workflows.

You can also use try Themer.

Themer enables you to create and switch themes from your *published* styles in your team library. Themer creates themes from sets of styles. Each set of styles should contain styles with the same name. When you make a selection, the plugin will identify styles which have matching names associated with the theme you selected, and apply them all at once; enabling you to do fast theme switches.