[Bug]: Custom themes custom CSS leftovers when selecting stock theme

### What happened?

When selecting a custom theme and then adding custom CSS, I would expect that reverting to a "stock" theme would ignore the custom CSS that was added when using a custom theme (or maybe was the feature designed to also be able to customize stock themes?). Nonetheless, there is a visual glitch when selecting the "System Default" option from the theme selector after applying custom CSS. The "Custom CSS is active" button/url remains visible when reverting to a stock theme. Furthermore, the custom CSS gets applied over the stock theme.

### How can we reproduce the issue?

I was able to reproduce the issue in the demo version of Actual following the steps below :

1. Select a custom theme and add custom CSS
2. Revert to any of the stock themes selection
3. See the "Custom CSS is active" button still being present.
4. When reverting to the "System Default" theme selection, the "Custom CSS is active" button is stacked below the "Light" theme dropdown selection.

### Where are you hosting Actual?

Docker

### What browsers are you seeing the problem on?

Chrome

### Operating System

Mac OSX