added Theming page

Theming.md

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+Trilium doesn't come (yet) with any built-in themes, but allows you to create custom CSS which is attached to the main HTML page.
+
+Just create a code note with CSS type, put your custom CSS code into the note's content and create "app_css" [[attribute|Attributes]]. When Trilium frontend starts, all notes with "app_css" attribute are appended in the style element of the Trilium HTML page.
+
+Once you made your changes, you can reload the Trilium frontend by pressing CTRL-R after which the changes will take effect.
+
+The reason why the CSS is stored in the (code) note instead of e.g. file in the Trilium folder is that as a note it can be automatically synced and versioned.
+
+## Night mode
+
+```CSS
+html, img, video {
+    filter: invert(100%) hue-rotate(180deg);
+}
+
+body {
+    background: black;
+}
+```
+
+[[images/black-css.png]]
+
+### Grey night mode
+
+Following grey CSS is less contrasty which might be easier on the eyes. The disadvantage is that it casts (barely) noticeable greyish color on the images.
+
+```CSS
+html {
+    filter: invert(90%) hue-rotate(180deg);
+}
+
+img, video {
+    filter: invert(100%) hue-rotate(180deg);
+}
+
+body {
+    background: #191819;
+}
+```
+
+[[images/grey-css.png]]