To adjust my WordPress sidebar font size, I had studied a day for CSS.
First, go to ‘Appearance -> Customize’ in ‘WP-admin’ page
Then Click ‘Additional CSS’
Type your codes like below. you can see the result in real time.
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If you want to adjust more options in your theme, Press F12 to open Devtools in Chrome.
Then, choose ‘select an element’
When you point your mouse cursor on a specific area, you can see debugged code.
For More Detailed Information. Always thanks for your contributions.