Make theme parsing more lenient (#7154)

This PR improves the theme parsing to be a bit more lenient, allowing
things like comments and trailing commas in theme files.

Release Notes:

- N/A

crates/theme/Cargo.toml

@@ -34,6 +34,7 @@ schemars = { workspace = true, features = ["indexmap"] }
 serde.workspace = true
 serde_derive.workspace = true
 serde_json.workspace = true
+serde_json_lenient.workspace = true
 serde_repr.workspace = true
 settings = { path = "../settings" }
 story = { path = "../story", optional = true }

crates/theme/src/registry.rs

@@ -259,7 +259,7 @@ impl ThemeRegistry {
                 continue;
             };
 
-            let Some(theme) = serde_json::from_reader(reader).log_err() else {
+            let Some(theme) = serde_json_lenient::from_reader(reader).log_err() else {
                 continue;
             };