FileZilla/FileZilla Pro comes with a number of icon sets that you can use. You can change the icon set used, or change the size of the icons.
To modify the icon set:
- In the main menu, click on Edit > Settings….
- Select Interface > Themes from the menu on the left.
- Select an icon set from the Theme drop down list.
- If desired, modify the scale to suit.
- Click on OK.