Remove table header background styling (v1.7.6)

Removed custom background colors from table headers to improve preview appearance and maintain consistency with normal theming.
## Changes
- Removed background-color from default theme table headers (was #f6f8fa)
- Removed dark theme table header background styling block
- Headers retain font-weight: 600 for visual distinction
- Table borders and alternating row colors remain intact
## User Request
User feedback: "remove the theming of table headers, it looks bad. Keep normal theming of preview"
🤖 Generated with [Claude Code](https://claude.com/claude-code)

package.json

@@ -1,6 +1,6 @@
 {
   "name": "pan-converter",
-  "version": "1.7.5",
+  "version": "1.7.6",
   "description": "Cross-platform Markdown editor and converter using Pandoc",
   "main": "src/main.js",
   "scripts": {