feat!: use the theme from lualine config (a string for filename or a table)

[Tyler-Barham]

Adds support to

• use a lualine theme that differs from the main theme (a string pointing to a file.lua)
• use a customized theme (a table of <mode>={ a={}, b={}, c={} })

[mawkler]

@Tyler-Barham Looks great! Thank you very much for the contribution!