fix(theme): improve contrast of search highlight text

[skirtles-code]

This PR changes the text color used for highlights within the local search. This is to improve the contrast in light mode.

The screenshot below shows:

1. First row - prior to feat(theme): improve color system #2797. Note that the background-color for highlights is the same for both light and dark mode.
2. Second row - current VitePress theme. The background-color is using the green theme, which varies between dark and light mode. In light mode, both the text and background are dark.
3. Third row - this PR. The color for the text in light mode is now white, for greater contrast with the background.

I've used --vp-c-neutral-inverse for the text. I'm unclear if this is the correct variable, but it gives the desired result of black or white, depending on the current theme mode.