feat: allow to set background color on outlined button

[fabiiomiiguel]

Fixes #

• Customise background color of outlined buttons.

Proposed Changes

• Add a new variable button-outlined-background-color (or another that you like more).
• Allow to set the background color of outlined buttons, like #ffffff instead of transparent.

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (26e90af) 62.83% compared to head (01c7ddc) 62.83%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #514   +/-   ##
========================================
  Coverage    62.83%   62.83%           
========================================
  Files           79       79           
  Lines         5527     5527           
  Branches      1535     1535           
========================================
  Hits          3473     3473           
  Misses        1944     1944           
  Partials       110      110           

Flag	Coverage Δ
oruga	65.96% <ø> (ø)
oruga-next	48.70% <ø> (ø)

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