The screenshots show the initial appearance of the themes but this is just the starting point as most of the themes support light and dark color modes, etc. and other details can be changed using CSS custom properties (CSS variables).

Amelia

Colorful "orphaned" Bootswatch theme

Neubrutiki

Implementation of neubrutalism design ideas

Boosted

Based on Orange Boosted Bootstrap theme

BookStacked

Adaptation of Open Source BookStack.app theme

Zephyr

Newer Bootswatch theme, since Tiki 25 (2022)

Morph

Neomorphism Bootswatch theme, since Tiki 23 (2021)

Quartz

New Bootswatch theme, since v.23 in 2021, with modified readability for Tiki

Vapor

Newer Bootswatch theme, since Tiki 23 (2021)

Pulse

Bootswatch theme, since Tiki 19 (2018)

TikiCorp

Dramatic corporate appearance

Default

The default Bootstrap appearance

Lighty

Lighter color palette emphasizing readability

Sketchy

Bootswatch theme, since Tiki 19 (2018)

Minty

In Tiki since Tiki 19 (2018), another Bootswatch theme

Solar

Bootswatch theme added in Tiki 19 (2018)

Materia

Bootswatch theme added in Tiki 19 (2018)

Lux

Bootswatch theme added in Tiki 19 (2018)

Sandstone

Bootswatch theme added in Tiki 14 (2015)

Yeti

By Bootswatch, since Tiki 13 (2014)

United

By Bootswatch, since Tiki 2012 (2014)

Superhero

Bootswatch theme in Tiki since version 13 (2014)

Spacelab

Theme by Bootswatch, in Tiki since version 13 (2014)

Slate

A Bootswatch theme — Tiki 13 (2014)

Simplex

Bootswatch theme, added to Tiki13 (2014)

Lumen

A Bootswatch theme — Tiki 13 (2014)

Journal

By Bootswatch, since Tiki13 (2014)

Flatly

By Bootswatch, since Tiki13 (2014)

Darkly

By Bootswatch, since Tiki13 (2014)

Cyborg

By Bootswatch, since Tiki13 (2014)

Cosmo

By Bootswatch, since Tiki13 (2014)

Cerulean

Bootswatch theme

Utopias

Originally created for attainable-utopias.org

Greenvalley

Bringing some nature to the website

Ohia

Inspired by ohia trees, which are the first to take root when lava cools

jQui

Originally, to showcase JQuery UI themes

FiveAlive

Eight flavors for websites

FiveAlive-lite

A visually lighter version of FiveAlive

Dark Shine

Design by Jack Qiao for OpenWebDesign

Business

Somewhat retro business theme

Strasa

Origins in a Strasbourg, France TikiFest

Tikinewt

Muted legacy theme

TheNews

Serif font and borders between columns for a news website look

Feb12

Colored side column and gradients