use $input-bg for $custom-select-bg

Created by: m5o

• use $input-bg for $custom-select-bg
• fixes readability on dark themes like e.g. 'darkly'
  • https://bootswatch.com/darkly/index.html#forms
  • https://bootswatch.com/slate/index.html#forms
  • https://bootswatch.com/superhero/index.html#forms
  • https://bootswatch.com/solar/index.html#forms

/cc @thomaspark