Use light/dark theme based on system preference (#17051)

Add a new default theme `auto`, which will automatically switch between
`gitea` (light) and `arc-green` (dark) themes depending on the user's
operating system settings.

Closes: #8183

3 files changed, 4 insertions, 4 deletions

diff --git a/docs/content/doc/advanced/config-cheat-sheet.en-us.md b/docs/content/doc/advanced/config-cheat-sheet.en-us.md
index 7929f35c57..de9b794db2 100644
--- a/docs/content/doc/advanced/config-cheat-sheet.en-us.md
+++ b/docs/content/doc/advanced/config-cheat-sheet.en-us.md
@@ -174,9 +174,9 @@ The following configuration set `Content-Type: application/vnd.android.package-a
 - `FEED_PAGING_NUM`: **20**: Number of items that are displayed in home feed.
 - `GRAPH_MAX_COMMIT_NUM`: **100**: Number of maximum commits shown in the commit graph.
 - `CODE_COMMENT_LINES`: **4**: Number of line of codes shown for a code comment.
-- `DEFAULT_THEME`: **gitea**: \[gitea, arc-green\]: Set the default theme for the Gitea install.
+- `DEFAULT_THEME`: **auto**: \[auto, gitea, arc-green\]: Set the default theme for the Gitea install.
 - `SHOW_USER_EMAIL`: **true**: Whether the email of the user should be shown in the Explore Users page.
-- `THEMES`:  **gitea,arc-green**: All available themes. Allow users select personalized themes.
+- `THEMES`:  **auto,gitea,arc-green**: All available themes. Allow users select personalized themes.
   regardless of the value of `DEFAULT_THEME`.
 - `THEME_COLOR_META_TAG`: **#6cc644**:  Value of `theme-color` meta tag, used by Android >= 5.0. An invalid color like "none" or "disable" will have the default style.  More info: https://developers.google.com/web/updates/2014/11/Support-for-theme-color-in-Chrome-39-for-Android
 - `MAX_DISPLAY_FILE_SIZE`: **8388608**: Max size of files to be displayed (default is 8MiB)
diff --git a/docs/content/doc/advanced/customizing-gitea.en-us.md b/docs/content/doc/advanced/customizing-gitea.en-us.md
index fe18a801f0..040dcfd3e8 100644
--- a/docs/content/doc/advanced/customizing-gitea.en-us.md
+++ b/docs/content/doc/advanced/customizing-gitea.en-us.md
@@ -321,7 +321,7 @@ A full list of supported emoji's is at [emoji list](https://gitea.com/gitea/gite
 
 ## Customizing the look of Gitea
 
-The default built-in themes are `gitea` (light) and `arc-green` (dark).
+The default built-in themes are `gitea` (light), `arc-green` (dark), and `auto` (chooses light or dark depending on operating system settings).
 The default theme can be changed via `DEFAULT_THEME` in the [ui](https://docs.gitea.io/en-us/config-cheat-sheet/#ui-ui) section of `app.ini`.
 
 Gitea also has support for user themes, which means every user can select which theme should be used.
diff --git a/docs/content/doc/help/faq.en-us.md b/docs/content/doc/help/faq.en-us.md
index 953aa9f012..e9551bbb71 100644
--- a/docs/content/doc/help/faq.en-us.md
+++ b/docs/content/doc/help/faq.en-us.md
@@ -158,7 +158,7 @@ Use [Fail2Ban]({{< relref "doc/usage/fail2ban-setup.en-us.md" >}}) to monitor an
 
 ## How to add/use custom themes
 
-Gitea supports two official themes right now, `gitea` and `arc-green` (`light` and `dark` respectively)  
+Gitea supports three official themes right now, `gitea` (light), `arc-green` (dark), and `auto` (automatically switches between the previous two depending on operating system settings).
 To add your own theme, currently the only way is to provide a complete theme (not just color overrides)
 
 As an example, let's say our theme is `arc-blue` (this is a real theme, and can be found [in this issue](https://github.com/go-gitea/gitea/issues/6011))