Dear Rockbox'ers,
I tried several themes from the gallery, but I could not find one that I enjoyed using on a daily basis. So I created my own, and I have now released it in the Rockbox gallery. The "source" is actually on Google Code as well, where it will be kept up to date.
Source code on GitHub:
https://github.com/danielweck/Rockbox-theme-OptimalInfoGallery link (Sansa Fuze v1):
http://themes.rockbox.org/index.php?themeid=1624&target=sansafuzeGallery link (Sansa Fuze v2):
http://themes.rockbox.org/index.php?themeid=1624&target=sansafuzev2Gallery link (iPod Color/Photo):
http://themes.rockbox.org/index.php?themeid=1624&target=ipodcolorGallery link (iRiver H3XX):
http://themes.rockbox.org/index.php?themeid=1624&target=iriverh300Gallery link (Philips GoGear HDD63x0):
http://themes.rockbox.org/index.php?themeid=1624&target=gogearhdd6330(note that the scrolling lines in the screendumps are displayed with jagged fonts due to how Rockbox captures the LCD data, the text is perfectly smooth in real life)
OptimalInfo is a high-contrast coloured theme with minimalist graphics, which displays an optimal amount of information using an easy-to-read 9-line layout (or 10 lines + 2 extra blank spacing for taller displays) based on a legible font. Layout tricks such as alternating line text, as well as separate screens for volume and seek controls, are used to maximise usability whilst reserving screen real estate for high-priority information.
The 9-line layout is composed of:
1) a red-ish heading at the top, alternating between the date/time and the battery level
2) the track number (out of total playlist items)
3) track title
4) track album
5) track artist
6) full file path (dimmed)
7) audio codec details (dimmed)
8) playback progress information (current time / track duration, percentage, remaining time)
9) progress bar
Additionally, there are discrete vertical volume level indicators on either side of the screen (live peak meters for the left and right audio channels).
The main screen, playing (note the stereo peak meters):

Main screen, paused (note the white highlight to indicate the playback state)(also note that coincidentally, the red-ish heading at the top is displaying the battery level instead of the date/time):

A separate screen is displayed when the volume level is changing (wheel action):

A separate screen is displayed during seeking (rewind, fast-forward):

The menu style is in keeping with the "while playing screen", nothing fancy (note that the image background is actually not used in the WPS, a similar gradient is drawn instead):

My Sansa is red, thus why I chose a similar hue in the color scheme:


I hope you enjoy it as much as I do!
Cheers, Daniel
Note about porting to other devices:Porting the theme to other devices should be quite easy. Simply pick a font that enables 9 lines of text to fit within the vertical screen space, and adjust the pixel width of the viewports in the WPS (main, volume control, seeking). For example, if your device's display is 240 pixels wide instead of 220px, add the difference (20px) to the width specified in the WPS.
As a matter of fact, I have already ported OptimalInfo to the Toshiba Gigabeat F/X platform, and it is in daily use on my F40 and F60 devices ;) (I find the huge font "35-Nimbus.fnt" very usable in practice, as I don't mind horizontal text scrolling) I had to tweak a couple of other things though, for example the playback information above the progress bar is shifted below, centred, and reduced (only "time/total"). I also implemented similar alterations in the dedicated "seek" screen (rewind / fast-forward), by splitting the information onto several lines.
It's all on the Google Code page, but not in the Gallery just yet. When ready, I will post a separate thread on the forum.
UPDATE: http://forums.rockbox.org/index.php/topic,34473.0.htmlUPDATE 2 (iPod Video 5G, Cowon D2): http://forums.rockbox.org/index.php/topic,34493.0.html