Steps to reproduce

Updated from node-sass 4.5.3 to 4.8.3

Which browser?

Chrome, possibly others

What is expected?

No change in how themes are applied

What is actually happening?

Theme seems to be broken, specifically for background colors. Processed CSS with node-sass 4.8.3:

.md-button.md-theme-default.md-primary {
    color: #1a237e;
    color: var(--md-theme-default-primary-on-background, #1a237e);
}

Reverting to node-sass 4.5.3 corrects the problem.