fix: code tab hover color in light mode (minor a11y issue)

[patricknelson]

When hovering over a <code-tabs> button while in light mode, the hover state flips from the primary text color, which is nearly black (#2c3e50) on a light gray, to white, which isn't visible on the light background. This is due to the failover which is currently being used since --code-button-focus-color is currently undefined. Since there's a light/dark theme here and it seems like the inherited text color is already used, I just set the current color to 75% opacity (which was what was already happening, particularly on the dark theme). So, no visible change for dark, just apply the same change for white.

Note: I'm using color-mix() here but support for it is very good and growing (~92% which is pretty decent). See: https://caniuse.com/mdn-css_types_color_color-mix. Not to mention it's more of a focus/hover state which won't affect touch devices so much (where the bulk of support is lacking, older Safari versions).

Before/after demo for you:

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