Go to Edit > Preferences then to the Add-ons tab. Click Install.

Choose the downloaded zip and click Install Add-on.

After the installation is complete it will show up in the add-on list. Enable the add-on by ticking the checkbox.

Once the add-on is enabled, all theme settings are consolidated in the Quick Theme panel (N-Panel > Q-Theme tab).

Toggle the Auto Theme feature to change the theme whenever you restart or load a blend file. A new accent color is picked automatically and applied on startup.

Pick an accent color with the color picker or hex field. The color is instantly propagated across the interface: selected buttons, menus, tooltips, text fields, sliders, list items, pie menus and panels all follow the accent.

Adjust Roundness to control the edge rounding of every widget at once, from sharp corners to fully rounded.

The Viewport and Outliner sub-panels are linked to the interface color by default: active and selected objects, vertices, edges and faces are derived from the accent color automatically.

Click the link icon in the sub-panel header to unlink and set the colors manually: active and selected objects, vertex, edge and face selection colors for the viewport; active, selected highlight and text colors for the outliner.

The accent color also drives the File and Asset Browser selection, UV/Image Editor element selection, Properties match and active modifier, animation editors (current frame, selected strips in the NLA and Sequencer), the playhead, and the default annotation color.