Thank you for posting here in the Community, @Miriam162.
You can only customize your invoice. It will help you change the font to a bigger size and the form's color.
I'm here to show you how:
Go to the Gear icon.
Choose Custom Form Styles from the pop-up window.
Locate the invoice template and hit Edit under the Action column.
Under the Design tab, tick the Make logo Edits section.
Click the Plus sign icon and select the file.
Hit Save, then Done.
Then, to change the invoice's color, click Splash on some color, then select the desired color.
In the meantime, once you change the file type of your logo, upload it again. I'd suggest reaching out to your IT personnel or the one who creates the logo. They know how to manage it.
I'm adding articles for more details about logo and transaction templates: