Thinking about how this plugin works

[AlisterH]

Hi, it is not obvious what the button is for. If you keep the button you should change the tooltip to "Refresh themes list". Ideally you wouldn't need a button, and the list should be refreshed any time a theme is added, renamed, or deleted.
However:

• themes are not automatically updated e.g. when a layer is turned on or off.
• so it does not make sense to display the most recently selected theme all the time, like you do. (In fact, it seems that when opening a project the first theme in the list is automatically selected in the drop-down, even though it likely doesn't reflect the current state of the project).
• so this plugin doesn't really make any sense. For it to make sense it would need to detect when a layer is added, removed, renamed or toggled on or off, and clear the selection in the dropdown, so that the user can see that the current state of the project does not match the theme.

I note users can just as easily (2 clicks) select a theme using the button already built-in to QGIS.

For what it's worth, I think a plugin that would be more useful would be one that provides a drop-down to select the style for a layer. This would mean you could change styles without switching to the Style Manager tab in the Layer Styling panel, or going through two levels in the Layers panel. And it would make sense to display a style all the time, because every layer always has a style active. The only thing is you would need to refresh the whole thing every time a different layer is selected in the layers panel.