If a user interface has an undesired look (elements not having padding or margins when it would be appropriate, etc.) and I implement a correction in a new (named) source control branch, would it be appropriate to name the branch as a bugfix or group it with other bugfix branches (and if not, what would be the most logical word[s] for grouping)? The cause isn't exactly a bug, as it isn't due to an error in code, stylesheets, layout files, etc., but the fix doesn't really seem to count as a new feature or enhancement either.
Wikipedia describes a software bug as "A software bug is an error, flaw, failure, or fault in a computer program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways."
As an undesired look can be classified as an incorrect result or unintended behaviour of the UI, a change to correct that look would certainly be a bugfix.
Another way to look at it is that the software contains a bug if it does not work as specified.
If those UI elements were specified to have no padding and no margins (and that can be traced back to a desire from the client), then your change isn't a bugfix because the look was 'as specified'. At best you have implemented a change request and at worst your change is incorrect and undesired.
For naming your branch, there is no harm in naming it as a bugfix. If it makes a difference for the development process that your team uses, you could double-check with your team lead/product owner/project manager if this fix was the result of a change request by the client or if it should be treated as a bugfix.