[BUG] Bold text does not render on Windows — Microsoft YaHei lacks Medium (500) weight #25975

Description

On Windows, OpenCode uses font-weight: 500 for bold/medium-weighted UI text (via the CSS variable --font-weight-medium: 500). The system default CJK font, Microsoft YaHei, only ships with two weights — Regular (400) and Bold (700) — and has no Medium (500) weight at all.

As a result, Windows users see no visual difference between normal and "bold" text throughout the entire OpenCode UI. This affects:

• UI elements — labels, headings, and other emphasized content render identically to regular text.
• AI output — Markdown bold syntax (**text**) in assistant replies is rendered with font-weight: 500, which also fails to produce any visual bold effect. Users cannot distinguish bold portions in AI responses at all.

This affects the Desktop app on any Windows system with CJK locale or Microsoft YaHei as the configured font.

Plugins

None

OpenCode version

v1.14.39 (Desktop, Windows x64)

Steps to reproduce

1. Install OpenCode Desktop on Windows (any version with Microsoft YaHei as the system CJK font — which is the default for Chinese locales).
2. Launch OpenCode.
3. Observe that bold/medium-weighted text (labels, headings, etc.) looks identical to regular-weight text — no visual bold effect is applied.

Screenshot and/or share link

Operating System

Windows 11

Terminal

Desktop App