fix: accept any AbstractDict in _normalize_theme

JSON.jl v1 returns JSON.Object{String,Any} from parsefile, which is
AbstractDict but not Dict. The previous `::Dict` signature dispatched
fine on JSON 0.21 (which returned Dict) but failed with MethodError on
v1. Relaxing to AbstractDict keeps the function working on both — the
inner `attrs isa Dict` guard is widened to `isa AbstractDict` for the
same reason.

Required to land #15 (CompatHelper: bump compat for JSON to 1).
Validated locally: 982/982 tests pass on both JSON 0.21 and 1.6.

Author: s-celles

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Apply JuliaFormatter v2.3.0 (blue style) across the codebase. Whitespace-only changes — no behavior change. The `Format Check` workflow was added recently and only runs on pull requests, so the v1→v2 style drift on `main` had gone unnoticed until CompatHelper PRs (#13, #14, #15) tripped the check.
+- `_normalize_theme` accepts any `AbstractDict` (previously `Dict` only). This lets `load_theme` work with JSON.jl v1, whose `parsefile` returns `JSON.Object{String,Any} <: AbstractDict` rather than `Dict{String,Any}`. Tests pass against both JSON 0.21 and 1.6.
 
 ## [0.5.3] - 2026-05-13
 

src/themes.jl

@@ -277,10 +277,10 @@ $(SIGNATURES)
 Normalize a raw parsed dictionary into a Theme.
 Ensures all inner values are Dict{String, Any}.
 """
-function _normalize_theme(raw::Dict)::Theme
+function _normalize_theme(raw::AbstractDict)::Theme
     theme = Theme()
     for (section, attrs) in raw
-        if attrs isa Dict
+        if attrs isa AbstractDict
             theme[string(section)] = Dict{String,Any}(string(k) => v for (k, v) in attrs)
         end
     end