DOCUMENTATION

docs/USER_GUIDE.md

@@ -131,9 +131,21 @@ Clicking on the cog icon in the upper right will go to the settings screen.
 
 ## Theme
 
-Here you can change, load and save the look of your Pleroma frontend. There are a lot of options, but there is a preview. Play around with the settings and see what you like!
+You can change the look and feel of Pleroma Frontend here. You can choose from several instance-provided presets and you can load one from file and save current theme to file. Before you apply new theme you can see what it will look like approximately in preview section.
 
-TODO: Maybe expand?
+Themes engine was made to be easy to use while giving an option for powerful in-depth customization - you can just tweak colors on "Common" tab and leave everything else as is.
+
+If there's a little check box next to a color picker it means that color is optional and unless checked will be automatically picked based on some other color or defaults.
+
+For some features you can also adjust transparency of it by changing its opacity, you just need to tick checkbox next to it, otherwise it will be using default opacity.
+
+Contrast information is also provided - you can see how readable text is based on contrast between text color and background, icons under color pickers represent contrast rating based on [WCAG](https://www.w3.org/TR/2008/REC-WCAG20-20081211/#visual-audio-contrast-contrast) - thumbs up means AAA rating (good), half-filled circle means AA rating (acceptable) and warning icon means it doesn't pass the minimal contrast requirement and probably will be less readable, especially for vision-challenged people, you can hover over icon to see more detailed information. *Please note* that if background is not opaque (opacity != 1) contrast will be measured based on "worst case scenario", i.e. behind semi-transparent background lies some solid color that makes text harder to read, this however is still inaccurate because it doesn't account that background can be noisy/busy, making text even harder to read.
+
+Apart from colors you can also tweak shadow and lighting, which is used mostly to give buttons proper relief based on their state, give panes their shade, make things glow etc. It's quite powerful, and basically provides somewhat convenient interface for [CSS Shadows](https://developer.mozilla.org/en-US/docs/Web/CSS/box-shadow).
+
+Another thing you can tweak is theme's roundness - some people like sharp edges, some want things more rounded. This is also used if you want circled or square avatars.
+
+Lastly, you can redefine fonts used in UI without changing fonts in your browser or system, this however requires you to enter font's full name and having that font installed on your system.
 
 ## Filtering