When printing, the icons in "show icon" blocks are rendered without sufficient contrast

When printing, the icons in "show icon" blocks are rendered without sufficient contrast. The 'on' pixels in the icon should be rendered in black instead of white.

To Reproduce
Steps to reproduce the behavior:

1. Drag the 'show icon (heart)' block into the 'on start' block.
2. Click gear icon, select printing.
3. Wait for the print preview dialog to show.
4. Inspect the print preview.

Expected behavior
The heart icon should be black ('on' pixels) and gray ('off' pixels).
Now the 'on' pixels of the icon are white, which is not distinguishable from the background.

Screenshots

micro:bit version (please complete the following information):
Not applicable.

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Firefox and Edge
• Version: makecode.microbit.org version = 4.0.18

Smartphone (please complete the following information):
Not applicable. Anyway, the fixed code should be tested on smartphones if printing is possible.

Additional context
None