To target that dropdown and any others like that, you can use
select {font-family: inherit; font-size: inherit;}
You can even put the color in there, but I don’t recommend it because changing the background color or the border or the border-radius all result in losing the little arrow.
If you want the rule to only affect select in widgets, instead make it
.widget select {font-family: inherit; font-size: inherit;}
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