themes: theme changes aren't rebuilt by `pxt serve`

When running `pxt serve` in a target folder, when I make a change to theme css, (for example changing a color in the target's `theme/color-theme/overrides/<target>-default-overrides.css`), the cli will report "rebuilding css" and "css build complete", but the changes are not reflected in the target after reloading.

Workround: restarting `pxt serve` loads the new css.

