There are 3 locations for Style Library:
1) Default Location: Design Data Folder, complete Styles
2) Template, complete Styles
3) Any other Inventor Files: ipt, iam, idw, partial Styles
The error message is saying the Style in the file you are editing is different then the Style in Default Location.
A quick fix is save the change to Default Library by select "Save Style to Style Library".
This will remove conflict on that computer. Everyone else opening you file will have the conflict again.
It is because you modified the Standard ANSI Library. For each Template you changed the Style, it is a modified copy of the Default. You will never get rid of the error message.
The best solution is create a new Standard based on ANSI (or ISO, whichever you want to use), ie Company Name ANSI.
For each style you change: Dimension, Balloon, Layer, Text etc, create a new one before you change it. I created new one for every styles so nothing is share with Default ANSI Standard.
Make sure the new standard use the new styles then "Save Style to Style Library".
Now you have 2 complete Styles: one in Style Library, one in Template.
If you use different Styles in different Templates, you should create new Standard for each.
It is a long and painful way but once its done, you'll have a proper company standard you can share from the Template file. It will also survive Inventor update, since updating Inventor will force you to use the new updated Default Style Library. Unless you have changed the folder.