Consistent style for lh-warnings

[exterkamp]

Mentioned in #6989. I think the lh-warnings style looks out of place with the report style e.g.

My suggestion is to replace the 1px border with a light orange background (#fff5e3):

Would love some thoughts. @hwikyounglee

[paulirish]

@yuinchien is looking at this.

[hwikyounglee]

Su^per late -

@exterkamp 's suggestion on using the bg color instead of border looks good to me while we're currently looking at a longer term approach in a separate effort.

[exterkamp]

I'm gonna give this to @hoten as part of the report-redesign 😄