Hi @lindsey.h ,

Welcome to the Community!

Unfortunately we don't currently have a functionality that would universally edit an email across all existing flows or allow you to change the branding (fonts and styles) of all of your messages from one place if you are not using universal blocks. We now offer universal blocks that you can set up and then if you use these universal blocks in your emails, you can just change it from one email and it should reflect in other templates that are using the same universal block. Here is a guide on how to do this: How to create and manage universal and saved content.  
 

When you hover over or click a block within an email template, you will see a star icon. Once you have a block formatted the way you would like it, click the star icon to save it as a universal block. Make sure to use a descriptive name so the content is easy to find later.

To access and reuse a universal block or section, open the Content tab and click Universal.

Then, you’ll be prompted to choose a universal block or section. Only universal content that was built in the new editor will be available to choose from. Search by the content’s name or scroll through all your universal content to choose one.

Once you find the block or section you would like to reuse, drag it into your template. 

To edit a universal content block or section, click on it from any template. You have two options for editing universal content:

• Edit across all instances
  Choose this option if you’d like your edits reflected in every template containing this piece of content
• Unlink and edit independently
  Choose this option if you’d like your edits to only apply in the template you’re currently editing

If you click Edit Across All Instances, editing other blocks in your template will temporarily be disabled. Make any desired edits to the universal content, then click Publish Changes to apply the changes across all instances of the content in other templates. Alternatively, use the back arrow to discard your changes and return to the main editor.

Use the # emails and templates button to see a list of all messages where the block is currently in use.

After you publish the changes, other blocks in the current template will become editable again. 

 
If your emails were setup previously without universal blocks, then you would have to change the font and styles individually by going into each email template. But by implementing the universal blocks, you can then edit the blocks from just one email and this should then reflect in all emails that has that same universal block as mentioned above.
 
I do apologize for the limitations on our end but hope this helps,

Alex