Implement omb theme

[RobLoach]

This introduces a themes plugin which allows changing the theme on the fly. This mimics the themes oh-my-zsh plugin.

Usage

theme <theme_name> - Changes the theme to specified theme.

theme - Changes the theme to some random theme.

lstheme - Lists installed themes.

[akinomyoga]

Thank you for the PR! I have a question. How is the upstream OMZ themes plugin different from omz theme [1]?

• [1] https://github.com/ohmyzsh/ohmyzsh/blob/f4423ebd09fbc7670815c3c20cc86c81b7319e5f/lib/cli.zsh#L658-L796

Since this is related to the OMB specific feature for themes, I feel this should be implemented as a subcommand of the omb command similar to omz theme. However, the upstream OMZ seems to contain quite similar features in two locations, plugins/themes and omz theme, which appears to be weird to me.

edit: If we switch to omb theme, you are going to replace the following stub code in lib/cli.bash:

oh-my-bash/lib/cli.bash

Lines 20 to 22 in e712342

	function _omb_cmd_theme {
	echo 'Not yet implemented'
	}

[RobLoach]

Namespacing it within omb makes so much sense. Thanks for linking the stub.

[RobLoach]

Not sure how I feel about manipulating the .bashrc file with omb theme set zork.

[akinomyoga]

_omb_module_require_theme is kind of "include once"; it checks whether the module is already loaded and loads the module only when it hasn't been loaded. So, with this implementation, one cannot switch to the theme that was loaded before.

To handle this, we may need to factorize a part of _omb_module_require (oh-my-bash.sh) into _omb_module_search and use it to obtain the location of the theme file.