update(docs): improve docs for extending an existing theme
update(docs): improve docs for extending an existing theme

Changing the default text color (black/white) on buttons is a common question that keeps coming up.
Hopefully this will help people better understand how to do that.

Closes #7973

acoreyj/material,Splaktar/material,imjoeco/material,daniel-nagy/material,acoreyj/material,HarrisCorp/material,imjoeco/material,ilovett/material,SamDLT/material,Frank3K/material,HarrisCorp/material,SamDLT/material,dmitriz/material,colinskow/material,profoundry-us/material,miguelcobain/material,miguelcobain/material,graingert/material,HipsterZipster/material,ilovett/material,HarrisCorp/material,welkinhealth/material,ErinCoughlan/material,SamDLT/material,gkalpak/material,david-gang/material,miguelcobain/material,j3ski/material,vertazzar/material,oostmeijer/material,ServiceManagementGroup/material,programmist/material,daniel-nagy/material,angular/material,Frank3K/material,HipsterZipster/material,bluebirdtech/material,marekmicek/material,ErinCoughlan/material,marekmicek/material,DevVersion/material,bluebirdtech/material,ServiceManagementGroup/material,DevVersion/material,graingert/material,pwistbac/material,programmist/material,programmist/material,david-gang/material,bluebirdtech/material,Frank3K/material,epelc/material,ilovett/material,vertazzar/material,welkinhealth/material,hodeyp/material,marekmicek/material,profoundry-us/material,ServiceManagementGroup/material,profoundry-us/material,colinskow/material,HarrisCorp/material,crisbeto/material,clshortfuse/material,ErinCoughlan/material,gkalpak/material,miguelcobain/material,oostmeijer/material,gkalpak/material,Frank3K/material,j3ski/material,imjoeco/material,programmist/material,j3ski/material,pwistbac/material,hodeyp/material,profoundry-us/material,acoreyj/material,clshortfuse/material,pwistbac/material,DevVersion/material,HipsterZipster/material,daniel-nagy/material,hodeyp/material,imjoeco/material,SamDLT/material,Splaktar/material,dmitriz/material,angular/material,hayloft/material,gkalpak/material,marekmicek/material,oostmeijer/material,acoreyj/material,welkinhealth/material,epelc/material,crisbeto/material,oostmeijer/material,colinskow/material,ServiceManagementGroup/material,welkinhealth/material,bluebirdtech/material,epelc/material,ErinCoughlan/material,vertazzar/material,epelc/material,Splaktar/material,angular/material,david-gang/material,graingert/material,hayloft/material,vertazzar/material,clshortfuse/material,HipsterZipster/material,colinskow/material,graingert/material,Splaktar/material,DevVersion/material,angular/material,dmitriz/material,crisbeto/material,hayloft/material,hayloft/material,david-gang/material,crisbeto/material,pwistbac/material,daniel-nagy/material,dmitriz/material,hodeyp/material,clshortfuse/material,ilovett/material,j3ski/material