Made the checkerboard background be more flexible in terms of scale and coloring. Provides dark theme for image diff. I suppose these colors should not be re-used for a color-picker for a example, because it's usually more convenient to always have it in the light mode. ## Test * go to https://next.forgejo.org/image-test/image-diff/commit/e6d3623c7e0277e09cf9a19278b1259840fb268799d6b65100904ed82d6b05d2 * or migrate https://next.forgejo.org/image-test/image-diff to your local instance ## Before/after  (Old any - New Forgejo dark - New Gitea dark - New Forgejo/Gitea light) --- Gradient property is taken from [here](https://www.reddit.com/r/css/comments/u08pf3/how_to_make_a_checkerboard_using_background/). [CSS compatibility](https://developer.mozilla.org/en-US/docs/Web/CSS/gradient/conic-gradient#browser_compatibility): about four years of browser versions. Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3870 Reviewed-by: Mai-Lapyst <mai-lapyst@noreply.codeberg.org>