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.


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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

src/utils/theme.go

@@ -0,0 +1,11 @@
+package utils
+
+import "os"
+
+func GetThemeFromEnv() string {
+	theme := os.Getenv("THEME")
+	if theme == "" {
+		theme = "auto"
+	}
+	return theme
+}