Here is the item again:
With her heavy dictionary,
(A) Donna
thumps the top of her ancient computer
monitor. (B)
Whenever the color tint flickers
(C) from
normal to sickly yellow.
You correctly realized that the period after monitor creates a fragment. Whenever the color tint flickers from normal to sickly yellow is a subordinate clause, which cannot stand alone as a complete sentence.
To fix this fragment, you chose to do this:
With her heavy dictionary, Donna thumps the top of her ancient computer
monitor whenever the color tint flickers from
normal to sickly yellow.
You have correctly employed Punctuation Rule 1:
Main Clause + Ø + Subordinate Clause.
Rock on, dude!