Update docs re: new themes in the IDE

Added documentation around support for new themes in the Web IDE, and
support for the dark theme for all of the Web IDE.

doc/user/profile/preferences.md

@@ -55,6 +55,11 @@ The default syntax theme is White, and you can choose among 5 different themes:
 
 ![Profile preferences syntax highlighting themes](img/profile-preferences-syntax-themes.png)
 
+[Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2389) in 13.0, the theme
+you choose also applies to the [Web IDE](../project/web_ide/index.md)'s code editor and [Snippets](../snippets.md).
+The themes are available only in the Web IDE file editor, except for the [dark theme](https://gitlab.com/gitlab-org/gitlab/-/issues/209808),
+which applies to the entire Web IDE screen.
+
 ## Behavior
 
 The following settings allow you to customize the behavior of GitLab's layout

doc/user/project/web_ide/index.md

@@ -43,6 +43,20 @@ you can find a more complete list of supported languages in the
 NOTE: **Note:**
 Single file editing is based on the [Ace Editor](https://ace.c9.io).
 
+### Themes
+
+> [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2389) in GitLab 13.0.
+
+All the themes GitLab supports for syntax highlighting are added to the Web IDE's code editor.
+You can pick a theme from your [profile preferences](../../profile/preferences.md).
+
+The themes are available only in the Web IDE file editor, except for the [dark theme](https://gitlab.com/gitlab-org/gitlab/-/issues/209808),
+which applies to the entire Web IDE screen.
+
+| Solarized Light Theme                                         | Dark Theme                              |
+|---------------------------------------------------------------|-----------------------------------------|
+| ![Solarized Light Theme](img/solarized_light_theme_v13.0.png) | ![Dark Theme](img/dark_theme_v13.0.png) |
+
 ## Commit changes
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/4539) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 10.4 and [brought to GitLab Core](https://gitlab.com/gitlab-org/gitlab-foss/issues/44157) in 10.7.