add function to get CSS-style RGB string attribute
add function to get CSS-style RGB string attribute

tbretl/PrairieLearn,rbessick5/PrairieLearn,tbretl/PrairieLearn,jakebailey/PrairieLearn,tbretl/PrairieLearn,mwest1066/PrairieLearn,parasgithub/PrairieLearn,parasgithub/PrairieLearn,tbretl/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,tbretl/PrairieLearn,parasgithub/PrairieLearn,jakebailey/PrairieLearn,jakebailey/PrairieLearn,parasgithub/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,jakebailey/PrairieLearn,parasgithub/PrairieLearn,rbessick5/PrairieLearn