Fix CSS tweaking when theme contrast is not default (< 10)
sp_tweak_background_colors: background-image rules were rewritten with an extra image() in the value. Removing the extra image() fixes the red background on affected UI elements (mainly buttons) when contrast is set to < 10.
ThemeContext::add_gtk_css: Replace some nullptrs with empty strings to avoid assertion failures.
The above fixes #5434 (closed). Also fixes #5431 (closed) and #5521 (closed), which look like duplicates of #5434 (closed).
ThemeContext::add_gtk_css: When loading the CSS for a theme, the variant should be set by whether the current theme is actually dark, not whether it's preferred dark. Fixes broken colours when contrast < 10 and the actual darkness of the theme does not match the preferDarkTheme setting (for example, when the theme only has a dark option regardless of what the user prefers).