activescaffold/active_scaffold — comment on commit 686f43c1fbd76224be9b32505ecde6d553a0d26d
File:  line 0
By scambra:

active_scaffold_colors cannot be required or variables cannot be overrided. active_scaffold can be imported or required. If you require active_scaffold you won't be able to override variables.

I didn't want to force using Sass, but it wasn't work for you. I don't want to force using sass in your css, but sass gem will be needed.
Can you try again? I have renamed active_scaffold.css.erb to active_scaffold.css.scss.erb and now it must work with require