Log warning instead of panicking on invalid color

fixes #498

src/theme.rs

@@ -10,8 +10,14 @@ macro_rules! load_color {
         $theme
             .as_ref()
             .and_then(|t| t.$member.clone())
-            .map(|c| Color::parse(c.as_ref()).expect(&format!("Failed to parse color \"{}\"", c)))
+            .and_then(|c| Color::parse(c.as_ref()))
-            .unwrap_or($default)
+            .unwrap_or_else(|| {
+                warn!(
+                    "Failed to parse color in \"{}\", falling back to default",
+                    stringify!($member)
+                );
+                $default
+            })
     };
 }