Add @steps-background and @process-icon-text-color variables for customizing step theme

[megawac]

@steps-background was added to make the background transparent. The @component-background default didn't make any sense in our theme
@process-icon-text-color was added to support showing the number when the background was transparent (otherwise would be showing a circle of @primary-color)

[mention-bot]

@megawac, thanks for your PR! By analyzing the history of the files in this pull request, we identified @YuhangGe, @afc163 and @PinkyJie to be potential reviewers.

[codecov]

Codecov Report

Merging #5624 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5624   +/-   ##
=======================================
  Coverage   86.75%   86.75%           
=======================================
  Files         183      183           
  Lines        4514     4514           
  Branches     1308     1308           
=======================================
  Hits         3916     3916           
  Misses        598      598

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