Share your Custom Themes (CSS) here

[clemzarch]

Custom CSS rules can be added in UMiBO's settings:

Click to view image

To change the default colors, paste this in the text area:

:root {
    --bg: #f0f0f0;
    --color: #333333;
    --faded-color: #595a5b;
    --hi: #cccccc;
    --hi-click: #a3d4ff;
    --field: #f9f9fb;
    --shadow: #00000015;
}

and change the hexadecimal color values.

To simplify this process I recommend right-clicking the background of the new tab page, and selecting "Inspect Element".
A panel on the right should list the rules currently applied. Clicking on the little circle next to an hexadecimal value opens a color selector:

Click to view image

Once you're done choosing the colors, copy the new values with the selector (:root { and the closing }) and paste them in the settings.

bg is the color of the background of the page as well as the background of the windows.
color is the color of the text
faded-color is the color of the search tips as well as the little cog icon
hi is the highlight color that appears when hovering over a link, a folder or other interactable elements
hi-click is the same thing but when you click the folder or link
field is the secondary background color, used on folders and window borders
shadow is the color of the shadows under each windows

[clemzarch]

as a first rudimentary example, here are some rules that try to ape Windows 95:

:root {
    --bg: #12818c;
    --color: #000;
    --faded-color: #fff;
    --hi: #00b;
    --hi-click: #00b;
    --field: #c1c4c9;
}

.window{
box-shadow: 15px 15px 0 #0003;
border-radius: 0;
}
.window main {
background: #fff;
}

.window .border,
.window .resize,
.window .border span,
.desktopFolder
 {
border-radius: 0;
border: 1px outset var(--field);
border-top: 1px solid #fff;
border-left: 1px solid #eee;
outline: 1px solid #0005;
}

.desktopLink:hover{
color: #fff;
border-radius: 0;
}