Btw., see the line 30 at:

https://pagure.io/fork/praiskup/copr/copr/blob/generate-colorscheme-always/f/frontend/coprs_frontend/generate_colorscheme

You see

cat <
@base: $opt_base_color;
@lighten: lighten(@base, 10%);

But there actually is:

cat <<EOF | ( echo "$warning" ; lessc -x - ) | tee $opt_static_files/css/copr-flavor.css
@base: $opt_base_color;
@lighten: lighten(@base, 10%);

So it seems that not only diff UI is broken.

I believe this is fixed in https://pagure.io/pagure/pull-request/3729

Hm, seems that PR isn't, it's fixing diff not PRs

This is fixed in https://pagure.io/pagure/pull-request/3733