The accusative of lunar and solar nouns is impossible
I think the hardest thing of this language so far has been trying to understand the complex endings that has every noun depending on the gender. I don't understand how the lunar noun "zokla" has "zokli" for plural and then "zokle" for singular accusative and "zoklī" for plural accusative, but then the noun "atroksia" has "atroksie" for singular and "atroksī" for both plural and plural accusative. I mean, they could have explained this better, because it's not so easy, and you can't move on, in the language if you don't know the genders. It's so frustrating!
P.S: Can somebody explain what the heck is a theme vowel?
This is actually covered by the first and second tables here https://wiki.dothraki.org/High_Valyrian_Noun_Declensions#Lunar-type:_vala However, it does appear that these "subtypes" are to this one example and the third declension.
So in your example, you can think of the roots as "zokl-" and "astroksi-." They are followed by "a" for sg. nom, "e" for sig. acc, "i" for pl. nom, and "ii" for pl. accusative.
It's best to think of ī as "ii" as essentially the only thing irregular here is the writing as "atroksī" is used instead of "atroksi-i".
I specifically covered this topic in a post I wrote last week located here: https://www.reddit.com/r/HighValyrian/comments/6qy3pg/understanding_gender_plural_and_accusative/
I'll repeat some of that here: Gender is HV is mainly around what consonant the word ends in, not what vowel, but sometimes genders behave int heir own way. The system is fairly regular, though. Therefore, you can't usually identify the gender of the word just based on what its last vowel is, but whatever the last vowel is will cause a predictable pattern in declension usually regardless of gender. (Unless the gender has its own special behavior in a certain situation.) In other words, the gender generally doesn't affect how the vowels change.
For words whose final vowels are a/e/y, you make the plural by changing that last vowel to i. Forming the accusative works the same way (also with i), except ☾ lunar singular nouns ending in -a will have -e as their accusative ending, and words whose last vowel is -e will end with -ī in accusative. Plural+accusative results in -ī, because it's like compounding two i's. You can imagine in your mind that i + i = ī. ☉ solar nouns have nominative plurals that instead of using -i, you repeat the last vowel (zaldīzesse, loktyssy), but in accusative it is regular.
Nouns whose last vowel is i/o don't have accusative forms, and their plural is made with -a. Not much fuss with these. ☉ solar nouns also do their nominative plural thing, but without repeating the last vowel (ēngossa, tubissa).
So on the wiki what is called "theme vowel" is the last vowel in a word's nominative form. The genders don't behave uniquely in all cases, only in particular cases, so when a declension is regular, what affects declension is mainly just the vowel regardless of what the gender is.