Add default values to _ide_theme_overrides

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

.ide.theme-dark is currently added to the top of the
_ide_theme_overrides.scss file 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.

This MR adds default values to all the var() instances, so
that that scoping may be removed.

app/assets/stylesheets/page_bundles/ide_themes/README.md

@@ -32,19 +32,7 @@ To add a new theme, follow the following steps:
 3. Copy over all the CSS variables from `_dark.scss` to `_solarized_dark.scss` and assign them your own values.
    Put them under the selector `.ide.theme-solarized-dark`.
 4. Import this newly created SCSS file in `ide.scss` file in the parent directory.
-5. To make sure the variables apply to to your theme, add the selector `.ide.theme-solarized-dark` to the top
-   of `_ide_theme_overrides.scss` file. The file should now look like this:
-
-   ```scss
-   .ide.theme-dark,
-   .ide.theme-solarized-dark {
-     /* file contents */
-   }
-   ```
-
-   This step is temporary until all CSS variables in that file have their
-   default values assigned.
-6. That's it! Raise a merge request with your newly added theme.
+5. That's it! Raise a merge request with your newly added theme.
 
 ## Modifying Monaco Themes