Default rulers color to red

Currently if the theme a user is using doesn't have `ui.virtual.rulers`
set and they set up a ruler it just fails silently making it really hard
to figure out what went wrong. Did they set incorrectly set the ruler?
Are they using an outdated version of Helix that doesn't support rulers?

This happened to me today, I even switched to the default theme with
the assumption that maybe my theme just doesn't have the rulers setup
properly and it still didn't work.

Not sure if this is a good idea or not, feel free to suggest better
alternatives!

helix-term/src/ui/editor.rs

@@ -20,7 +20,7 @@ use helix_core::{
 use helix_view::{
     document::{Mode, SCRATCH_BUFFER_NAME},
     editor::{CompleteAction, CursorShapeConfig},
-    graphics::{CursorKind, Modifier, Rect, Style},
+    graphics::{Color, CursorKind, Modifier, Rect, Style},
     input::KeyEvent,
     keyboard::{KeyCode, KeyModifiers},
     Document, Editor, Theme, View,
@@ -170,7 +170,20 @@ impl EditorView {
         theme: &Theme,
     ) {
         let editor_rulers = &editor.config().rulers;
-        let ruler_theme = theme.get("ui.virtual.ruler");
+        let default_style = Style {
+            bg: Some(Color::Red),
+            ..Default::default()
+        };
+        let ruler_theme = theme
+            .try_get("ui.virtual.ruler")
+            .map(|style| {
+                if let None = style.bg {
+                    default_style
+                } else {
+                    style
+                }
+            })
+            .unwrap_or(default_style);
 
         let rulers = doc
             .language_config()