This setting can only be edited by an admin. Access to branding to edit a theme is available from plan essential.
To customize the appearance of the documents to fit your needs you can create themes.
Go to Settings > Branding and click the Edit Theme button.

Once clicked, you will see a New Theme button in the top right.

You can also edit the existing themes by clicking on their name in the column.
What you can customize when creating or editing a theme
A window will pop up asking you where you can edit the following at the moment

Name
Give your theme a name
Background color
Changes the background color of the email header banner
Accent color
Changes the color of the View document button shown to the recipient in the email and also the color of the sign/approve button.
Text color
This changes the text color of the View document and Sign/Approve document button.
Background image
Changes the background behind the document when a recipient opens the document.
We recommend using a high-resolution background image.
Background type, fixed or scroll
Sign button text
(This changes the signing button text that the signers will click on to sign the document.
Default Editor Font
This changes the text font of the text that follows under the heading in the editor block.
Default Editor Heading Font
Changes the text font of the heading when writing an editor block
When choosing the editor fonts you will always see a preview of how the chosen Font will look to the right of the two columns.

We recommend using a high-resolution picture for your background image. When finished, click Save and close the window.
Use theme
To use the edited theme, go back to Settings > Branding and click on the theme. You can choose standard themes or your own created ones.
The theme you choose will affect all send-outs that all users do on the entity.
