Difficulty: ★☆☆☆☆
ISSUE
JDownloader2 GUI fonts are tiny in default. We will fix that in this tutorial.
SOLUTION
I hope you have installed and preconfigured via JDownloaderHeadless tool JDownloader app.
Now Open JDownloader and navigate to:
Settings > Advanced Settings
Confirm popup.
Search: font
Change values:
LAF.Settings: Font Scale Factor 120
LAF.Settings: Font Respect System DPI remove 
mark
LAF.Settings: Font Name Noto Sans
FIX FONTS LOOKING
Following ArchWiki and just set environment variable
_JAVA_OPTIONS="-Dawt.useSystemAAFontSettings=lcd_hrgb -Dswing.aatext=true"
Save it this line in /etc/environment file.
Reboot.
THEME
Dark theme used JDownloader-2-Dark-Theme