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

[nihil2501]

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Draft 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".