docs: fix input text color to match theme in custom form guide (#21750)
docs: fix input text color to match theme in custom form guide (#21750)

- without this, it's black on dark grey
andrewseguin/material2,DevVersion/material2,crisbeto/material2,DevVersion/material2,mmalerba/material2,angular/components,josephperrott/material2,angular/components,crisbeto/material2,crisbeto/material2,DevVersion/material2,josephperrott/material2,josephperrott/material2,andrewseguin/material2,andrewseguin/material2,mmalerba/material2,DevVersion/material2,mmalerba/material2,crisbeto/material2,andrewseguin/material2,mmalerba/material2,josephperrott/material2,angular/components,andrewseguin/material2,josephperrott/material2,DevVersion/material2,angular/components,mmalerba/material2