Fix theme coloring for components that use overrides e.g. accordion summary

[garrettjstevens]

This PR:

• updates theme.ts to get rid of an any and a @ts-ignore
• passes the provided theme to the overrides in createJBrowseTheme so that things that are styled by the overrides (e.g. the accordion summary) get the correct theme color

Accordion not being the right color noted by @scottcain here: https://gitter.im/GMOD/jbrowse2?at=6335cacc3a42316d331451a9

[codecov]

Codecov Report

Merging #3231 (7eba34b) into main (6ceeac5) will increase coverage by 0.00%.
The diff coverage is 77.77%.

@@           Coverage Diff           @@
##             main    #3231   +/-   ##
=======================================
  Coverage   59.40%   59.40%           
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  Files         672      672           
  Lines       28773    28775    +2     
  Branches     7001     7001           
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+ Hits        17093    17095    +2     
  Misses      11405    11405           
  Partials      275      275           

Impacted Files	Coverage Δ
packages/core/ui/theme.ts	84.61% <77.77%> (+1.28%)	⬆️

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[cmdcolin]

awesome :) can confirm fix visiting https://jbrowse.org/storybook/lgv/theme_ts_and_fixes/?path=/story/theming--page