activity-date-grouping text size tiny

In 11 sites(actually 10.2) the styles defining that from an OOTB perspective were localized to the activity story stream widget.  So it appears you still have some customizations either theme or widget wise overriding that style.  I am not sure what you mean by its getting knocked down, in browser tools does it show that is the active style or is it crossed out?  In chrome it should tell you  by crossing out styles that are being overriden.

I would agree there is a customization somewhere, but I can't seem to find it. Chrome shows the screen.less (14px) being overridden by the platform.less (9px). When I say "knocked down," I mean the size is smaller than it should be.

Where are all of the possible places that the CSS for Activity Story Stream would live?
(Remembering that this is a 10.1 instance that was then upgraded to 11.1 so the conversion to the 10.2 theme studio occurred automatically)

For more context, here is the site theme file widget-activity-story-stream.less:

	.activity-date-grouping {
		background-color: transparent;
		font-size: @font-size * 1.4;
		font-weight: @font-weight;
		color: @color-foreground;
		line-height: @line-height;
		margin: (@font-size * 1.5) 0 (@font-size * .5) 0;
		&.today {
			color: @color-foreground;
			background-color: transparent;
		}
	}

Here is the style.less from the Activity Story Stream Widget:

.activity-date-grouping {
		background-color: transparent;
		font-size: -evo-themeconfig-unit('baseFontSizeValue', 14px) * 1.4;
		font-weight: -evo-themeconfig-int('baseFontWeight', 400);
		color: -evo-themeconfig-color('foregroundColor', #11171A);
		line-height: -evo-themeconfig-unit('baseFontLineHeight', 1.5em);
		margin: (-evo-themeconfig-unit('baseFontSizeValue', 14px) * 1.5) 0 (-evo-themeconfig-unit('baseFontSizeValue', 14px) * .5) 0;
		&.today {
			color: -evo-themeconfig-color('foregroundColor', #11171A);
			background-color: transparent;
		}
	}

There are no CSS overrides defined in either group nor site themes so I'm really not sure where else to look.

(Is there a search all theme files for CSS/LESS function?)

Is this site publicly accessible

No, it's on our dev server at the moment. Interestingly, the size issue only happens on the home page. Other groups/pages with the Activity Stream Widget are displaying the text appropriately sized.

Is there perhaps a root group theme that's different than the site theme? If so, how does one get there?

Without being able to see this its not very easy to track down.  I don't see what you are seeing which means somewhere at some time something was customized or left in a state where this style was active.  Its really something you are going to have to track down with browser tools

I get that for sure, unfortunately, I'm out of ideas of where the customization could be living if it isn't in the site theme files (or at least the obviously named ones) or the widget style files. Can you provide any clues as to where those customizations could be hiding? Is there a way to deconstruct platform.less to determine which source file contributed the lines in use?

Did you have any system notifications or exceptions with issues processing LESS files?

No system notifications after the standard site upgrade notification was resolved. No exceptions processing LESS files (only exceptions are where I typoed in an unrelated widget edit.)

Are you using a custom theme?