Add theme support using environment variable (#145)

* Add theme support using environment variable

* Propagate theme variable to template in options.go

Propagate the `theme` variable from the environment to the template in `src/routes/options.go`

* Retrieve the `theme` variable from the environment using `os.Getenv("THEME")`
* Set the `theme` variable in the `gin.H` map when rendering the `home.html` template


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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/httpjamesm/AnonymousOverflow/pull/145?shareId=6397c9b4-9450-425c-bbbe-019425965d2b).

* Move all theme environment variable logic to a utils function

Move theme environment variable logic to a utils function and update routes to use it.

* Add `GetThemeFromEnv` function in `src/utils/theme.go` to derive the theme from environment variables and default to "auto" if not set.
* Update `src/routes/home.go` to import and use `GetThemeFromEnv` in the `GetHome` function.
* Update `src/routes/options.go` to import and use `GetThemeFromEnv` in the `ChangeOptions` function.
* Update `src/routes/question.go` to import and use `GetThemeFromEnv` in the `ViewQuestion` function.


---

For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/httpjamesm/AnonymousOverflow/pull/145?shareId=a0dab6f3-027c-4f6e-85fe-60e7675d0e70).

* fix: imports removed by copilot

* fix: override theme in posthome

* style: reduced repetition in themes with common vars

public/globals.css

@@ -1,9 +1,13 @@
 :root {
+    --code-bg: #36383d;
+    --code-fg: #ffffff;
+}
+
+:root,
+[data-theme="dark"] {
     --main-bg: #1b1f26;
     --text-color: #fff;
     --muted-text-color: #b3b3b3;
-    --code-bg: #36383d;
-    --code-fg: #ffffff;
     --input-bg: #2b303b;
     --input-bg-hover: #3b404b;
     --meta-bg: #525262;
@@ -12,12 +16,10 @@
 }
 
 @media (prefers-color-scheme: light) {
-    :root {
+    :root:not([data-theme="dark"]) {
         --main-bg: #dbdbdb;
         --text-color: #000;
         --muted-text-color: #636363;
-        --code-bg: #36383d;
-        --code-fg: #ffffff;
         --input-bg: #bcbcbc;
         --input-bg-hover: #a8a8a8;
         --meta-bg: #aaa8a8;
@@ -26,6 +28,17 @@
     }
 }
 
+[data-theme="light"] {
+    --main-bg: #dbdbdb;
+    --text-color: #000;
+    --muted-text-color: #636363;
+    --input-bg: #bcbcbc;
+    --input-bg-hover: #a8a8a8;
+    --meta-bg: #aaa8a8;
+    --divider-color: #b5b5b5;
+    --link-color: #335ad0;
+}
+
 a {
     color: var(--link-color);
 }