startplasma: Tweak the time when global themes are switched

Vlad Zahorodniirequested to merge
work/zzag/startplasma-mid-transition into master

Original commit message(s)

  • startplasma: Tweak the time when global themes are switched

Amends 4eb568e9.

CCBUG: 511973

Reason for the change

Test plan

Screenshots or screen recordings

Bugs fixed

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