Well, if you want to override it using another CSS, you could try using the
Head HTML code webpart (placed on your Master page) and use code like:
{% ViewMode == "Design" ? "<link rel=\"stylesheet\" type=\"text/css\" href=\"style.css\">" : "" %}
This macro will link the CSS only for Design tab. Therefore, if you are linking the original CSS before linking this one, it should get overridden for the Design tab.
Dave