[color] correclty set default color theme
[color] correclty set default color theme

Previously if --color option is set without the base theme (e.g.
`--color hl:56`), the default 16 color will be used as the base theme.
Now change it to dark256.

lotabout/skim,lotabout/fzf-rs,lotabout/skim,lotabout/skim