[FEATURE]: Custom themes can use calculated `system` colors

[nihil2501]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

PR

Description

OpenCode themes address UI concepts that don't necessarily exist in themes for other tools like Helix, e.g.:

• backgroundElement
• backgroundMenu
• backgroundPanel
• border
• borderActive
• borderSubtle

In my case, I wanted to make a custom theme that matches what I have in Helix. I also already have terminal ANSI colors that match my Helix theme. Rather than go through the manual work to choose colors for every slot OpenCode offers, I'd like to be able to customize some colors, while falling back to the values of the calculated system theme for the rest.

Example

This "extreme" example shows syntax highlighting I took from Helix's "Pop Dark" theme, and everything else OpenCode is deriving given that Ghostty's theme is set to "HaX0R Blue".

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Can't use custom themes that use ANSI colors #4236: Can't use custom themes that use ANSI colors
• Use terminal theme colors by default #101: Use terminal theme colors by default
• [FEATURE]: "system" theme to use existing terminal palette #3644: [FEATURE]: "system" theme to use existing terminal palette

These issues are all about using system/terminal colors as fallbacks in themes, which directly relates to your feature request for custom themes to use system colors. Feel free to ignore if your specific case differs from these.