Improve note style colour contrast

[garethrees]

BEFORE:

• Red: https://webaim.org/resources/contrastchecker/?fcolor=D32A0E&bcolor=FDE7E3 (4.31:1 – bad)
• Yellow: https://webaim.org/resources/contrastchecker/?fcolor=D3710E&bcolor=FDF0E3 (3.05:1 – bad)
• Green: https://webaim.org/resources/contrastchecker/?fcolor=358753&bcolor=CFECDA (3.51:1 – bad)
• Blue: https://webaim.org/resources/contrastchecker/?fcolor=006687&bcolor=EDFBFF (6.1:1 – pretty good)

AFTER:

• Red: https://webaim.org/resources/contrastchecker/?fcolor=8b1c0a&bcolor=FDE7E3 (7.81:1 – pass)
• Yellow: https://webaim.org/resources/contrastchecker/?fcolor=733e08&bcolor=FDF0E3 (7.74:1 – pass)
• Green: https://webaim.org/resources/contrastchecker/?fcolor=1f5031&bcolor=CFECDA (7.4:1 – pass)
• Blue: https://webaim.org/resources/contrastchecker/?fcolor=00526e&bcolor=EDFBFF (8.15:1 – pass)

Fixes https://github.com/mysociety/alaveteli/issues/8277.

Screenshots

BEFORE

AFTER

[skip changelog]