It's a weird issue. The border of a TextField has the color black (see Screenshot):

Checking the CSS in the developer console:

So obviously there is no color set on the border.
This happens in Chrome, but in Firefox it works. But it get even more weird: When I build my app with the flag NODE_ENV=production then it DOES work properly.
Also changing these two lines of code here: https://github.com/callemall/material-ui/blob/master/src/TextField/TextFieldUnderline.js#L94-L95
… to borderBottom: 'solid 2px ' + focusColor, work as well, even with a normal build.
I wanted to create a Webpackbin example, but there it works fine.
I'm using Meteor, don't know if that matters. But maybe someone find a solution.
See this minimal setup showing the problem:
bad zip file... github doesn't allow .tgz
Just use CRA and put a TextField in a minimal project.
This is happening to me too. No matter which theme you use, textfield underline is always black.
Also, this is happening in chrome, and it's working fine in firefox.
@lewisdiamond It seems your zip isn't complete. public and src folders are empty.
I think that it's a duplicate of #5697. Could you confirmed that it's solved on the master branch?
Oh, I didn't see that bug you linked although I actually tried to find that issue before opening a new one …
Anyway, it seems to work with current master (if I did everything right).
@vuhrmeister: great. I'll close this issue since it appears to be resolved, thank you for openning it!
That's quite an important issue. Let's release something before the end of the week.
Any insight into what caused this issue? I didn't even update any libraries, just did a clean install.
@rsolomon: it's probably the ^ (caret) on your Material-UI dependency in package.json, npm treats that as a _update to most recent major version_, so if you have a package with ^1.2.3 and it releases a 1.2.4, npm will automatically install it.
I'm still seeing this on 0.16.6 :(

Not sure what I need to change.
It appears border-color is set to initial, causing the issue.

@oliviertassinari This is still broken if you set {userAgent: 'all'} in a call to getMuiTheme, e.g., getMuiTheme({...}, {userAgent: 'all'});.
New issue created here: https://github.com/callemall/material-ui/issues/5902
@elTallarin I have the same issue as you, and only in Chrome when develop with webpack using HotModuleReplacement Plugin, have you found solution?
posting for same error, using webpack HMR, userAgent all and chrome
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That's quite an important issue. Let's release something before the end of the week.