Move some css variables from _ide_theme_overrides.scss file to ide.scss

What does this MR do?

This is part 1 of the fix of issue: #217432 (closed)

Move some css variables around in IDE

Move some css variables from _ide_theme_overrides.scss to ide.scss to remove redundant styles.

Screenshots

This MR's impact area is both the white theme and the dark theme, but there are no changes, visually. I've manually verified both the themes.

• The fact that there are no visual changes in the White theme can be verified by looking at each change in ide.scss and verifying that the color change to CSS variable includes the originally defined color as its default.
• Dark theme needs to be verified manually, which I've done for almost all the cases, which basically include:
  • all sidebars, 3 on the left (edit, review and commit) and 3 on the right (pipelines, live preview and terminal)
  • main area
  • footer
  • dropdowns