Damn, they don't rhyme? I will never get used to all these different vowel sounds in Danish...I have never seen this before, I have to admit, but I can explain what it could mean. The thing about "lys" and "kys" is that both words can be both singular, plural and imperatives of verbs. Hence:
Lys: "light" or "lights" or "glow" (imperative, "light" as an imperative sounds funny)
Kys: "kiss" or "kisses" or "kiss" (imperative)
I'd say the to most propable interpretations are 1) that it means "light and kisses", that is your friends sends you light and kisses or 2) that it means "glow and kiss", bot imperatives, your friend encourages you to radiate as a person and kiss others Personally, I think that the first one is the most likely.
As a curiosity, the words don't even rhyme
It's hard to explain, but it is true that lys and kys do NOT rhyme. I'd be interested to hear natives like Sepia and Andreas Jensen explain the phonetics here.
Kys: the "y" here is the short version (shorter sound with less energy, slightly farther back in the mouth. In addition, there is no glottal stop between the "y" and the "s."
Thus, the two words don't rhyme. To natives, the sounds probably are utterly dissimilar. Non-natives with a good ear will be able to hear the difference even if they can't reproduce it. Non-natives without a good ear will be able to neither hear nor reproduce the difference.
Just to make it clearer, why do the Danish spelling conventions not use the doubling of final consonants to render that the preceding vowel has a short quality? Lys and kyss, to be clear. This convention is that used in Swedish and Norwegian (with some, not many, exceptions). The alternative would be to double the vowel when long, like in the Dutch orthography. I know little Danish, but knowing the corresponding Swedish spelling of cognate words helps me in 80% of cases at least to make out the difference between short and long quality of the vowels.You can call me Andreas ;-)
I'd say the sounds are utterly dissimilar and very easily distinguishible, even for non-natives. The thing is that the "y" in "lys" is pronounced with the "basic" y-sound (the same as ü or a "u" in French) and with the glottal-stop as explained above.
"kys" is without the glottal stop and this changes the pronounciation of the vowel to an ø-sound (same as ö in German and Swedish).
So the vowel-sounds aren't even the same... I seem to recall that the lys/kys relationship is called a "bogstavrim" (lit. letter-rhyme) in Danish, in that by looking at the letters you'd think that the two words rhyme, so you can WRITE them together as a type of pseudo-rhyme... But when they are actually spoken they don't