Document Web IDE theming strategy

Add a README to the _ide_themes directory to guide future contributors
on how to add support for more themes in the Web IDE.

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

+# Web IDE Themes
+
+Web IDE currently supports 5 syntax highlighting themes based on themes from the user's profile preferences:
+
+* White
+* Dark
+* Monokai
+* Solarized Dark
+* Solarized Light
+
+Currently, the Web IDE supports the white theme by default, and the dark theme by the introduction of CSS
+variables.
+
+The Web IDE automatically adds an appropriate theme class to the `ide.vue` component based on the current syntax
+highlighting theme. Below are those theme classes, which come from the `gon.user_color_scheme` global setting:
+
+| # | Color Scheme    | `gon.user_color_scheme` | Theme class             |
+|---|-----------------|-------------------------|-------------------------|
+| 1 | White           | `"white"`               | `.theme-white`           |
+| 2 | Dark            | `"dark"`                | `.theme-dark`            |
+| 3 | Monokai         | `"monokai"`             | `.theme-monokai`         |
+| 4 | Solarized Dark  | `"solarized-dark"`      | `.theme-solarized-dark`  |
+| 5 | Solarized Light | `"solarized-light"`     | `.theme-solarized-light` |
+| 6 | None            | `"none"`                | `.theme-none`            |
+
+## Adding New Themes (SCSS)
+
+To add a new theme, follow the following steps:
+
+1. Pick a theme from the table above, lets say **Solarized Dark**.
+2. Create a new file in this folder called `_solarized_dark.scss`.
+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.
+
+## Modifying Monaco Themes
+
+Monaco themes are defined in Javascript and are stored in the `app/assets/javascripts/ide/lib/themes/` directory.
+To modify any syntax highlighting colors or to synchronize the theme colors with syntax highlighting colors, you
+can modify the files in that directory directly.
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