Fix setting of Dial background

[philippjfr]

Fixes #7178

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.25%. Comparing base (d5e2155) to head (9e9bd07).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/widgets/indicators.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7261   +/-   ##
=======================================
  Coverage   82.24%   82.25%           
=======================================
  Files         337      337           
  Lines       50293    50297    +4     
=======================================
+ Hits        41362    41370    +8     
+ Misses       8931     8927    -4     

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