I have found out the test 38 looks for 6 color appearence (you have 12, two times for each color) inside the linear-gradient(), wich means you have to use a shorthand method.
Wich does the same what you did, but with fewer code.
Example: var(--colorName) start% end%,
On a side note:
I noticed you have no borders:
--solid-border: 1% solid #000;
--dashed-border: 1% dashed #000;
Try to use other value like pixel.