Hi @mtnarwhal 
Just adding a tiny thing to the perfect and professional answer of @pavlos_kairis
Additinally to your theme switcher, you can use redshift (pre-installed on your UM). This software make the screen colors more red to avoid blue light. It is changing the gamma automaticly depending on your localisation and the day time.
My config : .config/redshift.conf
[redshift]
temp-day=5000
temp-night=4000
transition=1
location-provider=manual
adjustment-method=randr
[manual]
lat=0.304769
lon=2.742510
[randr]
screen=0
Don't forget to put your own latitude and longitude.
Playing those numbers will mqke your screen looks more red
temp-day=5000
temp-night=3500
Next just run redshift and enable it with the panel indicator.