API Management (APIM) has a new developer portal which is use-friendly and easy to customize. You can customize your UI for Developer portal by understanding it with help of Following Materials:
Some Steps to Customize the Developer Portal for changing the Logo and Text are explained in this blog.
Step 1: Click on any widget and click on Edit.
Step 2: Click on upload a file to change the logo.
Step 3: Select your newly uploaded logo. Now you can click on save at the bottom to save your changes on the developer portal.
Step 4: Select the Text you want to edit, and you can simply Type in the Text box and change its style, etc.
Step 5: After editing click on Save button on bottom page.