What you are pointing out about the Blue Opal theme is indeed correct. This is because, in this specific theme, the alternating color is calculated with the following less function:
@alt-background-color: contrast(@base, lighten(@background, 3%), lighten(@base, 3%), 0.5);
Or in other words, it checks the color contrast between the base and background colors and chooses the one with the bigger contrast. When you use certain colors, such as #8BAABF, the calculations from this function result in white color, which is why, both the normal and the alternating rows become white.
This is why the best approach for this case is to add a custom CSS rule targeting the .k-alt row only as you already have.
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