Some Slack themes that I use. I think they're pretty, you don't have to, but you can use them if you like.
Great question!
Below every preview image is a series of hex values. To add the theme to your Slack, open your team, open preferences (press cmd + comma on a Mac, no clue on Windows, but it should be somewhere easily accessible), and pick Sidebar Theme
option. You should see a button that says Feeling adventurous? You can customize your theme and share it with others.
You can paste the hex values in there, and voila, you have a custom theme.
##So without further ado, themes!
#D0D0D0,#A0A0A0,#E74C3C,#2C3A49,#19B5FE,#2C3E50,#4EEC91,#F39C12
#45474E,#3C3F45,#D05F4F,#E8E8E9,#5D6365,#D6D6DD,#3BC566,#5DAEE9
#3B4452,#45474E,#017ECC,#FEFEFE,#FFB925,#FEFEFE,#00D293,#FF4821
#222222,#333333,#E0E0E0,#222222,#888888,#FEFEFE,#EEF0F1,#34495E
#444A47,#333333,#262626,#FEF13E,#FC3935,#FFFFFF,#7FD1F5,#FC3935
#22A7F0,#3498DB,#FFCF4B,#303030,#2ECC71,#FEFEFE,#EA4C88,#3EDC81
#330A00,#400A00,#CE1822,#E8E8E9,#0187F2,#D6D6DD,#00D4FF,#00B0D4
Do whatever the hell you want, these are just hex values. Enjoy!