Custom theme causes panic due to missing markdown properties #488

Bug Report

Summary

Custom themes that don't include all required markdown properties cause a nil pointer dereference panic during theme initialization, making the application completely unusable.

Error Details

Caught panic:
runtime error: invalid memory address or nil pointer dereference
...
github.com/lucasb-eyer/go-colorful.MakeColor({0x0?, 0x0?})
        /home/runner/go/pkg/mod/github.com/lucasb-eyer/go-colorful@v1.2.0/colors.go:26 +0x20
github.com/sst/opencode/internal/styles.AdaptiveColorToString({{0x0?, 0x0?}, {0x0?, 0x0?}})
        /home/runner/work/opencode/opencode/packages/tui/internal/styles/markdown.go:317 +0x64
github.com/sst/opencode/internal/styles.generateMarkdownStyleConfig({{_, *}, {*, _}})
        /home/runner/work/opencode/opencode/packages/tui/internal/styles/markdown.go:41 +0x94

Root Cause

The crash occurs when generateMarkdownStyleConfig tries to create colors from theme properties that don't exist. When markdown properties are missing from a custom theme, the program attempts to create colors from nil/empty values, causing the panic in MakeColor.

Steps to Reproduce

1. Create a custom theme JSON file missing markdown properties (e.g., markdownText, markdownHeading, etc.)
2. Place it in a theme directory (e.g., ~/.config/opencode/themes/)
3. Launch opencode with this theme
4. Application immediately panics and crashes

Example Problematic Theme

Any theme that follows the documented schema but omits markdown properties will trigger this:

{
    "$schema": "https://opencode.ai/theme.json",
    "defs": {
        "black": "#21222c",
        "red": "#f9515d"
    },
    "theme": {
        "primary": { "light": "red", "dark": "red" },
        "background": { "light": "black", "dark": "black" }
        // Missing markdown properties causes panic
    }
}

Expected Behavior

• Application should provide sensible fallbacks for missing theme properties
• OR clearly document which properties are required vs optional
• OR gracefully handle missing properties without panicking

Actual Behavior

Application panics with nil pointer dereference, making it completely unusable.

Workaround

Add all required markdown properties to the theme:

"markdownText": { "light": "black", "dark": "white" },
"markdownHeading": { "light": "blue", "dark": "blue" },
"markdownLink": { "light": "cyan", "dark": "cyan" },
"markdownLinkText": { "light": "cyan", "dark": "cyan" },
"markdownCode": { "light": "green", "dark": "green" },
"markdownBlockQuote": { "light": "brightBlack", "dark": "brightBlack" },
"markdownEmph": { "light": "yellow", "dark": "yellow" },
"markdownStrong": { "light": "red", "dark": "red" },
"markdownHorizontalRule": { "light": "brightBlack", "dark": "brightBlack" },
"markdownListItem": { "light": "cyan", "dark": "cyan" },
"markdownListEnumeration": { "light": "cyan", "dark": "cyan" },
"markdownImage": { "light": "blue", "dark": "blue" },
"markdownImageText": { "light": "cyan", "dark": "cyan" },
"markdownCodeBlock": { "light": "white", "dark": "white" },
"diffHunkHeader": { "light": "brightBlack", "dark": "brightBlack" }

Environment

• opencode version: Latest (occurs across versions)
• OS: Multiple (reproduced on different systems)
• Theme: Custom theme following documented schema

Suggested Fixes

1. Add defensive programming: Check for nil/missing properties before calling MakeColor
2. Provide fallbacks: Use default colors when theme properties are missing
3. Improve documentation: Clearly specify which theme properties are required
4. Schema validation: Validate themes on load and provide helpful error messages

This is a significant usability issue as it makes custom themes completely unusable unless users know the undocumented requirement for all markdown properties.