Avoid affecting the theme

[microbit-matt-hillsdon]

The keyboard navigation plugin sets a theme in its constructor.

This affects at least the highlight colour in MakeCode:

I think @cpcallen previously explained to me that this was temporary / a workaround but I couldn't find anything tracking it so I've raised this issue. If I'm mistaken and it's an accessibility-motivated choice then let's discuss.

[cpcallen]

We're planning to introduce some kind of indicator of blocks having focus, and by default moving the cursor will both focus and select the block.

In the meantime, the colour of the cursor (and therefore selection) was changed to (a) match the colour of the cursor marker when positioned in other locations (e.g. on the connection between blocks in a stack), and (b) be more visible—because we had had feedback that the yellow outline was not always very visible (especially on colour picker blocks).

I think the main question of what this will eventually look like should be addressed in the context of focus indicators (perhaps for now in #142, but ultimately in core).

In the mean time: are you requesting that we remove this temporary change now? We can certainly do it, albeit at the cost of less consistency in the appearance of the cursor.