* Make loading placeholder colors match the current font color
Currently, the loading placeholder effect has a default gray color. However, since users can modify their themes with the Site Editor and choose a different set of colors for their websites, it would be interesting to make those placeholders match the color palette.
In this commit, the idea was to modify the `placeholder` mixin to replace the transparent font color with the current color and also modify the background-color and the linear-gradient to match the current font color. Furthermore, transparency was added to the middle color of the linear-gradient so we can keep the loading animation close to what it currently is.
* Add opacity to placeholder mixin
Before our changes, when the font color was dark, we had a lighter placeholder background color. After the changes the color is currently darker than before so the idea for this commit is to change the opacity of the placeholder in a way that the current color blends with the background color set for the theme.
* Change placeholder mixin opacity
After testing different combinations of colors, we decided to change the opacity to 0.15 so when the font color is darker the placeholder will have a lighter background color.
* bot: update checkstyle.xml
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