Add default values based on the white theme's colors to the variables in _ide_theme_overrides.scss

As a follow-up to #214702 (closed), based on #214702 (comment 335646731):

After !31241 (merged) is merged:

.ide.theme-dark is currently added to the top of _ide_theme_overrides.scss to scope the refactor work of moving to CSS variables to the dark theme only. So that other themes, especially the white theme remain unaffected.

We should add default values to all the var() instances, so that this scoping may be removed. Also remove the respective color, background-color, or border-color definitions from _ide.css so that that file may be cleaned up.