Add --theme CLI flag to override theme file path

[alycda]

Adds a new --theme (-t) command-line option that allows specifying a direct path to a CUSTOM theme file, similar to how --config works for the configuration file. This is useful for environments like Nix where avoiding symlinks and using CLI overrides is preferred.

The --theme flag takes precedence over the theme specified in the config file (whether custom or not) and supports theme inheritance via the 'inherits' field (which the config override file does not).

Usage: hx --theme /path/to/theme.toml file.txt

[alycda]

alternatively, the --config flag could be updated to point to a directory override instead of single file?

[alycda]

I tested with:

HELIX_RUNTIME="$(nix-store -q $(which hx))/lib/runtime" ~helix/target/release/hx --theme ./helix/helix-config/themes/mine.toml

as the runtime was needed so my local build could find the built-in theme files.

with CUSTOM theme:

inherits = "boo_berry"

"ui.background" = {}

"ui.cursor.primary.select" = { fg = "berry", bg = "bubblegum" }
"ui.cursor.primary.insert" = { fg = "berry", bg = "mint" }

and output (logs not committed):

DEBUG: Theme file specified: ./helix/helix-config/themes/mine.toml
DEBUG: Loading theme from file: ./helix/helix-config/themes/mine.toml
DEBUG: Searching for theme 'boo_berry' in directories:
DEBUG:   - ~/.config/helix/themes
DEBUG:   - ~/.config/helix/runtime/themes
DEBUG:   - /nix/store/xkqwkr94b9p123qjcy1kcdq0r6h88ijc-helix-25.07.1/lib/runtime/themes
DEBUG:   - ~/helix/target/release/runtime/themes

[the-mikedavis]

You can control the configured theme with -c/--config already. Including the theme in the package means including it in your runtime directory - historically you could put themes in ~/.config/helix/themes/*.toml but it should be within a runtime directory like ~/.config/helix/runtime / $HELIX_RUNTIME (or the compiled-in versions) instead. We want to keep the number of CLI flags small since Helix is interactive, so I'm not inclined to accept anything like this.

[alycda]

@the-mikedavis I understand not wanting to add another cli flag and you're correct that I can specify a non-custom theme in the --config file but I cannot specify a custom theme. I'm wondering if the --config flag should be used for a directory containing the files instead of the config file itself (apologies if this has already been discussed and rejected, I couldn't find any issues regarding that idea). if --config were a directory, then languages.toml could/should also be read from the directory override.

if I updated this pr with a new flag --config-folder might that be something you would accept? Keeping --config(-file) for backwards compatibility. I previously had the assumption that --config flag was for a folder since there are a handful of configuration files.