Make font more readable

Depending on the operating system, the font may end up less readable.
On Windows this is not that bad but it's visible.
Here the upper one is the Darko version and below it is the Thunderbird compose window.

Enlarged version:

Ideas to fix this:

• somehow increase font weight of all text
• change font for a more readable one
• change zoom level
• change font color to white (this may help a lot, but it's hard to tell how well it's gonna behave with existing filter)

I will experiment with different options. But I can't think of any good solution yet. Only the zoom level change sounds like the best solution here.