Mastodon (High Contrast) - It seems to improve text readability on screen Notifications, vs. Mastodon (Dark).

Mastodon (High Contrast):

Font Color:
RGB 194, 206, 222
Hex #c2cede
https://g.co/kgs/79rfYo5

Contrast Ratio: 13.17:1 (Pass)
https://webaim.org/resources/contrastchecker/?fcolor=C2CEDE&bcolor=000000

Mastodon (Dark):

Font Color:
RGB 96, 105, 132
Hex #606984
https://g.co/kgs/BYVzQRe

Contrast Ratio: 3.85:1 (Fail)
https://webaim.org/resources/contrastchecker/?fcolor=606984&bcolor=000000

Not perfect, but better